ci: upgrade to using macos-13

In April, GitHub announced that the `macos-13` pool is available:
https://github.blog/changelog/2023-04-24-github-actions-macos-13-is-now-available/.
It is only a matter of time until the `macos-12` pool is going away,
therefore we should switch now, without pressure of a looming deadline.

Since the `macos-13` runners no longer include Python2, we also drop
specifically testing with Python2 and switch uniformly to Python3, see
https://github.com/actions/runner-images/blob/HEAD/images/macos/macos-13-Readme.md
for details about the software available on the `macos-13` pool's
runners.

Also, on macOS 13, Homebrew seems to install a `gcc@9` package that no
longer comes with a regular `unistd.h` (there seems only to be a
`ssp/unistd.h`), and hence builds would fail with:

    In file included from base85.c:1:
    git-compat-util.h:223:10: fatal error: unistd.h: No such file or directory
      223 | #include <unistd.h>
          |          ^~~~~~~~~~
    compilation terminated.

The reason why we install GCC v9.x explicitly is historical, and back in
the days it was because it was the _newest_ version available via
Homebrew: 176441b (ci: build Git with GCC 9 in the 'osx-gcc' build
job, 2019-11-27).

To reinstate the spirit of that commit _and_ to fix that build failure,
let's switch to the now-newest GCC version: v13.x.

Signed-off-by: Johannes Schindelin <[email protected]>

.github/workflows/main.yml

@@ -276,11 +276,11 @@ jobs:
             pool: ubuntu-20.04
           - jobname: osx-clang
             cc: clang
-            pool: macos-12
+            pool: macos-13
           - jobname: osx-gcc
             cc: gcc
-            cc_package: gcc-9
-            pool: macos-12
+            cc_package: gcc-13
+            pool: macos-13
           - jobname: linux-gcc-default
             cc: gcc
             pool: ubuntu-latest

ci/lib.sh

@@ -253,11 +253,9 @@ ubuntu-*)
 	export PATH="$GIT_LFS_PATH:$P4_PATH:$PATH"
 	;;
 macos-*)
-	if [ "$jobname" = osx-gcc ]
+	MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python3)"
+	if [ "$jobname" != osx-gcc ]
 	then
-		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python3)"
-	else
-		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python2)"
 		MAKEFLAGS="$MAKEFLAGS APPLE_COMMON_CRYPTO_SHA1=Yes"
 	fi
 	;;