Don't require singlefilehost in host.native (dotnet#79565)

* Don't require singlefilehost in host.native

The corehost project currently tries to build the singlefilehost with
the embedded DAC, which we can skip if we never built the
singlefilehost in the first place.

* Update corehost.proj

Co-authored-by: Viktor Hofer <[email protected]>

Author: agocke

src/native/corehost/corehost.proj

@@ -44,7 +44,8 @@
             @(InjectResourceTool)"
     Outputs="$(SingleFileHostDestinationPath);$(SingleFileHostSymbolsDestinationPath)"
     DependsOnTargets="ResolveProjectReferences"
-    Condition="'$(RuntimeFlavor)' != 'Mono'">
+    Condition="'$(RuntimeFlavor)' != 'Mono' and
+               Exists('$(SingleFileHostPath)')">
     <Copy SourceFiles="$(SingleFileHostPath)" DestinationFiles="$(SingleFileHostDestinationPath)" />
     <Exec Condition="'$(TargetOS)' == 'windows'"
           Command="&quot;$(DotNetTool)&quot; exec @(InjectResourceTool) --bin &quot;$(DacPath)&quot; --image &quot;$(SingleFileHostDestinationPath)&quot; --name MINIDUMP_EMBEDDED_AUXILIARY_PROVIDER" />