Use compiler hash instead of volatile mtime for macOS ccache

The mtime is volatile in macOS jobs, causing cache misses.

As compiler binary is the same, use compiler's binary hash instead of its mtime.

See https://github.com/hendrikmuhs/ccache-action/issues/146

When upstream action is updated, this can be adapted

.github/workflows/ci.yml

@@ -181,6 +181,10 @@ jobs:
         uses: hendrikmuhs/ccache-action@v1
         with:
           key: ${{ matrix.cache_key }}
+      - name: Configure ccache
+        run: |
+          # See https://github.com/hendrikmuhs/ccache-action/issues/146
+          ccache --set-config=compiler_check=content
       - name: Install GCC problem matcher
         uses: ammaraskar/gcc-problem-matcher@master
       - name: Build OpenRCT2