Tighten NodeLaunchData.EnvironmentOverrides nullability to IDictionary<string, string?>?

src/Build/BackEnd/Client/MSBuildClient.cs

@@ -469,11 +469,10 @@ private bool TryLaunchServer()
                 // Set DOTNET_ROOT so the apphost server child can locate the runtime; this
                 // override is replaced by the client's environment on the first build command
                 // (see OutOfProcServerNode.HandleServerNodeBuildCommand → SetEnvironment).
-                // The `!` works around dotnet/msbuild#13761.
                 NodeLaunchData launchData = new(
                     MSBuildLocation: _msbuildLocation,
                     CommandLineArgs: string.Join(" ", msBuildServerOptions),
-                    EnvironmentOverrides: DotnetHostEnvironmentHelper.CreateDotnetRootEnvironmentOverrides()!);
+                    EnvironmentOverrides: DotnetHostEnvironmentHelper.CreateDotnetRootEnvironmentOverrides());
 
                 using Process msbuildProcess = nodeLauncher.Start(launchData, nodeId: 0);
                 _launchedServerPid = msbuildProcess.Id;

src/Build/BackEnd/Components/Communications/DetouredNodeLauncher.cs

@@ -144,7 +144,9 @@ public Process Start(NodeLaunchData launchData, int nodeId)
         /// <summary>
         /// Creates environment variables with optional overrides for app host bootstrap.
         /// </summary>
-        private static BuildParameters.IBuildParameters CreateEnvironmentVariables(IDictionary<string, string> environmentOverrides)
+#nullable enable annotations
+        private static BuildParameters.IBuildParameters CreateEnvironmentVariables(IDictionary<string, string?> environmentOverrides)
+#nullable disable annotations
         {
             var envVars = new Dictionary<string, string>();
             foreach (DictionaryEntry baseVar in Environment.GetEnvironmentVariables())

src/Build/BackEnd/Components/Communications/INodeLauncher.cs

@@ -25,7 +25,7 @@ internal readonly record struct NodeLaunchData(
         string MSBuildLocation,
         string CommandLineArgs,
         Handshake? Handshake = null,
-        IDictionary<string, string>? EnvironmentOverrides = null);
+        IDictionary<string, string?>? EnvironmentOverrides = null);
 
     internal interface INodeLauncher
     {

src/Build/BackEnd/Components/Communications/NodeLauncher.cs

@@ -287,7 +287,9 @@ private static STARTUPINFOW CreateStartupInfo(bool redirectStreams)
         /// (caller passes the inherited environment).
         /// </returns>
         [SupportedOSPlatform("windows")]
-        private static bool BuildEnvironmentBlock(ref ValueStringBuilder builder, IDictionary<string, string> environmentOverrides)
+#nullable enable annotations
+        private static bool BuildEnvironmentBlock(ref ValueStringBuilder builder, IDictionary<string, string?> environmentOverrides)
+#nullable disable annotations
         {
             if (environmentOverrides == null || environmentOverrides.Count == 0)
             {

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs

@@ -817,7 +817,7 @@ private NodeLaunchData ResolveAppHostOrFallback(
             {
                 CommunicationsUtilities.Trace($"For a host context of {hostContext}, using app host from {appHostPath}.");
 
-                IDictionary<string, string> dotnetOverrides = DotnetHostEnvironmentHelper.CreateDotnetRootEnvironmentOverrides(dotnetHostPath);
+                var dotnetOverrides = DotnetHostEnvironmentHelper.CreateDotnetRootEnvironmentOverrides(dotnetHostPath);
 
                 return dotnetOverrides == null
                     ? throw new NodeFailedToLaunchException(errorCode: null, ResourceUtilities.GetResourceString("DotnetHostPathNotSet"))

src/Framework/DotnetHostEnvironmentHelper.cs

@@ -99,7 +99,7 @@ internal static void ClearBootstrapDotnetRootEnvironment(IDictionary<string, str
             // Clear architecture-specific overrides that would take precedence over DOTNET_ROOT
             foreach (string varName in _archSpecificRootVars)
             {
-                overrides[varName] = null!;
+                overrides[varName] = null;
             }
 
             _cachedOverrides = overrides;
@@ -114,14 +114,14 @@ internal static void ClearBootstrapDotnetRootEnvironment(IDictionary<string, str
         /// </summary>
         /// <param name="environment">The environment dictionary to modify.</param>
         /// <param name="overrides">The overrides to apply. If null, no changes are made.</param>
-        internal static void ApplyEnvironmentOverrides(IDictionary<string, string> environment, IDictionary<string, string>? overrides)
+        internal static void ApplyEnvironmentOverrides(IDictionary<string, string> environment, IDictionary<string, string?>? overrides)
         {
             if (overrides is null)
             {
                 return;
             }
 
-            foreach (KeyValuePair<string, string> kvp in overrides)
+            foreach (KeyValuePair<string, string?> kvp in overrides)
             {
                 if (kvp.Value is null)
                 {