Use ccache on macOS

Originally (aseprite#2699) ccache was only enable for Linux due to suspiciously low
macOS yields.

I have addressed the likely issue upstream
(hendrikmuhs/ccache-action#147) and ccache can be used
on macOS now as well.

.github/workflows/build.yml

@@ -19,7 +19,7 @@ jobs:
         submodules: 'recursive'
     - name: ccache
       uses: hendrikmuhs/ccache-action@v1
-      if: runner.os == 'Linux'
+      if: ${{ runner.os == 'Linux' || runner.os == 'macOS' }}
       with:
         key: ${{ matrix.os }}-${{ matrix.enable_ui }}-${{ matrix.build_type }}
     - uses: turtlesec-no/get-ninja@main