rustbuild check does not support “keep going” (-k)

[jirutka]

When using --enable-rustbuild, make check -k does not run all tests regardless of failures.

#31322 (comment):

It seems it [rustbuild] doesn't support the -k (keep going) flag (that should be reported on the issue tracker if it's not there already), which should continue running the full test suite even if some individual test fails.

[eddyb]

cc @rust-lang/tools

[jirutka]

This is actually duplication of #40219, I found it just now.

[alexcrichton]

Indeed! Closing in favor of that.