CI: upgrade to macos-12, and pin OSX version

Per [1] and the warnings our CI is emitting GitHub is phasing in
"macos-12" as their "macos-latest".

As with [2], let's pin our image to a specific version so that we're
not having it swept from under us, and our upgrade cycle can be more
predictable than whenever GitHub changes their images.

1. https://github.com/actions/runner-images/issues/6384
2. 0178420b9c (github-actions: run gcc-8 on ubuntu-20.04 image,
   2022-11-25)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -240,11 +240,11 @@ jobs:
             pool: ubuntu-latest
           - jobname: osx-clang
             cc: clang
-            pool: macos-latest
+            pool: macos-12
           - jobname: osx-gcc
             cc: gcc
             cc_package: gcc-9
-            pool: macos-latest
+            pool: macos-12
           - jobname: linux-gcc-default
             cc: gcc
             pool: ubuntu-latest