Rewrite It_retries_on_failure_to_create_apphost test in AppHost.cs

[akoeplinger]

Found this in #48034 and disabled the test on Unix.

With the changes from dotnet/runtime#110417 we now delete the apphost if it exists.
This breaks the test since it tries to lock the file via FileShare.None to test that retries happen, but FileShare.None doesn't prevent a file from being deleted on Unix platforms, see dotnet/runtime#52700

sdk/test/Microsoft.NET.Build.Tests/AppHostTests.cs

Lines 461 to 463 in f2dd559

	var intermediateAppHost = Path.Combine(intermediateDirectory, "apphost" + Constants.ExeSuffix);
	using (var stream = new FileStream(intermediateAppHost, FileMode.Open, FileAccess.Read, FileShare.None))

The test likely needs to be rewritten.

FYI @jtschuster