Tweak coreclr test build script (#81975)

Tweak coreclr test build script (#81975)

1. No longer use `-priority=1` syntax: use `-priority 1` instead.
It was already documented this way, and probably works both ways on
Windows because batch scripts "eat" the `=` sign sometimes.
2. Fix a YML script to not pass the `=`. This is probably unused code
currently because we build tests in CoreCLR on Linux.
3. Add a few more `%MsgPrefix%` output lines.