"build coreclr" rebuilds everything every time

[dsyme]

Previously

build coreclr

was clean on a second build. Now it rebuilds everything, doing the build from source, making incremental build & test very slow for contributors

[dsyme]

Also build coreclr test debug doesn't seem to test the debug build cleanly, as IO see testbin\release... being used in the build:

C:\GitHub\dsyme\visualfsharp\Tools\dotnetcli\dotnet.exe C:\GitHub\dsyme\visualfsharp\tests\testbin\release\coreclr\fsc\fsc.exe --noframework ...

[dsyme]

Closing old issue, fixed by #4273 if not before