ci: re-enable ccache download/upload behind use_cache

if:false blocked OnePlus proven ccache flow; wire to inputs.use_cache (default true) as in 9fc52848

.github/workflows/main.yml

@@ -110,7 +110,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Download custom ccache once
-        if: false
+        if: inputs.use_cache
         run: |
           set -uo pipefail
           echo "::group::Download ccache"
@@ -131,7 +131,7 @@ jobs:
           fi
           echo "::endgroup::"
       - name: Upload ccache artifact
-        if: false
+        if: inputs.use_cache
         uses: actions/upload-artifact@v7
         with:
           name: ccache-binary