ci: Only run clang-format on ubuntu-stable GH runner

ci: Only run clang-format on ubuntu-stable GH runner

Previously we were running clang-format across multiple operating
system versions and hence clang versions, and it turns out
clang has changed the preferred formatting multiple times.

We could *probably* dig in and try to pin things more strongly
but eh...for now let's arbitrarily just use whatever's in
the default GH Action ubuntu-latest runner as that should
be equally painful for everyone.

Signed-off-by: Colin Walters <walters@verbum.org>