Fix timeouts in coreroot_determinism test in GC stress mode (#56770)

Fix timeouts in coreroot_determinism test in GC stress mode (#56770)

I have found two deficiencies in our CoreCLR test infrastructure
related to running Crossgen2 in GC stress mode; this change should
fix both:

1) In the Unix (bash) variant of Execute.Crossgen.targets we weren't
properly unsetting the GC stress-related COMPlus environment
variables.

2) In the particular case of the coreroot_determinism test this
needed fixing in r2rtest that is internally used by the test
implementation (on both Windows and Unix).

Thanks

Tomas