Fix bin\crystal.ps1 writing to standard error stream

[HertzDevil]

The bin\crystal.ps1 script itself writes to PowerShell's error stream, which is actually mapped to the output stream by default when the script is executed via powershell.exe -Command, rather than inside a PowerShell session. This PR adds the necessary workaround.

In particular, it makes make -fMakefile.win clean_cache work correctly on Windows. Without it the target would run:

if exist "Using compiled compiler at .\.build\crystal.exe C:\Users\nicet\AppData\Local\crystal\cache" rd /S /Q "Using compiled compiler at .\.build\crystal.exe C:\Users\nicet\AppData\Local\crystal\cache"