Use the installed dotnet runtime to exec the dll on MacOS instead of the CLI version

[dibarbet]

On Mac we use an dotnet executable dll instead of an actual executable (currently we only build on windows which doesn't allow us to create a codesigned executable for mac).

However, in this code path we were just launching 'dotnet' on the CLI instead of using the runtime downloaded dotnet. If you don't have .net7 available on the CLI, it'll throw (since the server requires the .net 7 runtime)

You must install or update .NET to run this application.

App: C:\Users\dibar\.vscode\extensions\ms-dotnettools.csharp-2.0.212-win32-x64\.roslyn\Microsoft.CodeAnalysis.LanguageServer.dll
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '7.0.0-preview.7.22362.8' (x64)
.NET location: C:\Program Files\dotnet\

The following frameworks were found:
  6.0.11 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  6.0.18 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

Learn about framework resolution:
https://aka.ms/dotnet/app-launch-failed

To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=7.0.0-preview.7.22362.8&arch=x64&rid=win10-x64

This can be reproduced on windows by only having .net 6 installed on the CLI and pointing the server path setting (dotnet.server.path) to a dll instead of the exe.

Resolves #5836