Remove pgrep dependency from KillTree implementation

[tmds]

KillTree has a dependency on pgrep:

msbuild/src/Utilities/ProcessExtensions.cs

Lines 49 to 52 in ec6ed83

	var exitCode = RunProcessAndWaitForExit(
	"pgrep",
	$"-P {parentId}",
	out string stdout);

This means it can't work on systems which don't have this tool installed.

dotnet-watch has a similar dependency which causes it to fail in certain container images: dotnet/aspnetcore#27950.

cc @Forgind

[JaynieBai]

Should we try catch exceptions here?

[rainersigwald]

@JaynieBai looks like the trend in this file is to catch + ignore exceptions:

msbuild/src/Utilities/ProcessExtensions.cs

Lines 23 to 27 in ec6ed83

	catch (InvalidOperationException)
	{
	// The process already exited, which is fine,
	// just continue.
	}

[Forgind]

I think the ideal solution here, rather than just catch and ignore an exception, is to find a suitable replacement for pgrep. We still want the process and all its descendants to die; we just can't rely on pgrep to exist.