ci: test on macOS 12 like libgit2 does

There seems to be some issue with Homebrew's ninja install on the newer versions
so let's stick to whatever libgit2 is testing with.

Author: carlosmn

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         ruby: [ '2.6', '2.7', '3.0', '3.1', '3.2' ]
-        os: [ macOS-latest ]
+        os: [ macOS-12 ]
     runs-on: ${{ matrix.os }}
 
     name: Ruby ${{ matrix.ruby }} on ${{ matrix.os }}