ci: Extend the msys2 testing to the "clangarm64" environment

ci: Extend the msys2 testing to the "clangarm64" environment

The clangarm64 msys environment doesn't have a "gcc" command
by default, unless the "gcc-compat" package is installed.

One may consider using "cc" as well, which is available when
the "toolchain" package is installed in each environment, but
that tool in clang environments fails the "direct" test case.