Use ccache in Mac CI builds

Install it using the same action we use for Unix builds and put the
directory containing its wrappers in the PATH.

Note that we need to do it after adding Homebrew directory to the PATH
because it isn't there by default on our self-hosted runner even if it
is on GitHub Actions ones.

.github/workflows/ci_mac.yml

@@ -151,6 +151,13 @@ jobs:
         fi
         echo "wxMAKEFILE_CXXFLAGS=$wxMAKEFILE_CXXFLAGS $error_opts" >> $GITHUB_ENV
 
+        echo "PATH=/opt/homebrew/bin:/opt/homebrew/opt/ccache/libexec:$PATH" >> $GITHUB_ENV
+
+    - name: Install CCache
+      uses: hendrikmuhs/ccache-action@v1.2
+      with:
+        key: ${{ matrix.name }}
+
     - name: Before install
       run: |
         ./build/tools/before_install.sh