DotNetSdkLocationHelper does not validate symlink target exists

[elinor-fung]

DotNetSdkLocationHelper looks for dotnet on the user's PATH. On non-Windows, it uses realpath to resolve symlinks, but does not check that the target exists.

MSBuildLocator/src/MSBuildLocator/DotNetSdkLocationHelper.cs

Lines 95 to 113 in d563efd

	// We will generally find the dotnet exe on the path, but on linux, it is often just a 'dotnet' symlink (possibly even to more symlinks) that we have to resolve
	// to the real dotnet executable.
	// This will work as often as just invoking dotnet from the command line, but we can be more confident in finding a dotnet executable by following
	// https://github.com/dotnet/designs/blob/main/accepted/2021/install-location-per-architecture.md
	// This can be done using the nethost library. We didn't do this previously, so I did not implement this extension.
	foreach (string dir in Environment.GetEnvironmentVariable("PATH").Split(Path.PathSeparator))
	{
	string filePath = Path.Combine(dir, exeName);
	if (File.Exists(Path.Combine(dir, exeName)))
	{
	dotnetPath = filePath;
	break;
	}
	}
	if (dotnetPath != null)
	{
	dotnetPath = Path.GetDirectoryName(isWindows ? dotnetPath : realpath(dotnetPath) ?? dotnetPath);
	}

This can end up passing a directory to hostfxr_resolve_sdk2 that does not represent a .NET install, resulting in no SDKs being found.

See dotnet/runtime#83469