.github/workflows/compilers.yml: Use GNUMAKEFLAGS.

[junaruga]

This PR comes from #4875 (comment) .

Because the make environment variable causes some rubygems tests to fail.
And to align with .cirrus.yml.

I also tested with check: true on my forked repo, as currently there is no test enabling check: true. And here is the CI result.

-          - { key: default_cc, name: gcc-11,    value: gcc-11,    container: gcc-11 }
+          - { key: default_cc, name: gcc-11,    value: gcc-11,    container: gcc-11, check: true }
...
-          - { key: default_cc, name: clang-14,  value: clang-14,  container: clang-14 }
+          - { key: default_cc, name: clang-14,  value: clang-14,  container: clang-14, check: true }

[junaruga]

The each case's total running time is around 4+ minutes. It's same level with the previous one.
https://github.com/ruby/ruby/pull/4880/checks?check_run_id=3672941483

[shyouhei]

:LGTM: !!

[junaruga]

Thanks for reviewing!