do_cmake:sh: use newer gcc if available

do_cmake:sh: use newer gcc if available

despite that we are using clang in `run-make-check.sh`, `do_cmake.sh`
is still used by some workflows like jenkins' ceph-pr-api job.
now that we've migrated to C++20, we need to use GCC-11 or up for
building the tree. GCC-11 is installed from PPA repo in
`install-deps.sh`, but to avoid interfere with the build of older
branches which do not use GCC-11, as their builds might break if
we use GCC-11 for building them. we don't use the alternative machinary
to point gcc to gcc-11, see 8f342a32ce4a236c45c67497ae34f508e032a9e1.

so, in this change, we try to use the newest GCC in system when
running `do_cmake.sh`.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>