To provide accurate coverage information.
It was originally RelWithDebInfo because MoveIt uses it
and Travis's timeout would kick in with the old setup.
According to Robert the timeout should not be a problem anymore with
the GHA ci setup. Actually I don't think the timeout was ever a problem for MTC,
but the config was just copied.
And add warnings for gcc AND clang builds (as they can differ slightly).
Clang does not support -Wused-but-set-parameter yet, though
support is [on the way](https://reviews.llvm.org/D100581).
So this gets rid of this warning we see in CI:
> error: unknown warning option '-Wno-unused-but-set-parameter'; did you mean '-Wno-unused-parameter'?