gitlab: just use plain --cc=clang for custom runner build

gitlab: just use plain --cc=clang for custom runner build

I think this was because older Ubuntu's didn't alias clang to whatever
the latest version was. They do now so lets use that and not break.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230124180127.1881110-4-alex.bennee@linaro.org>