ci: only save cache on master builds

.github/workflows/test_rust.yml

@@ -65,6 +65,7 @@ jobs:
         uses: Swatinem/rust-cache@v2
         with:
           shared-key: "desktop"
+          save-if: ${{ github.ref == 'refs/heads/master' }}
 
       - name: Check formatting
         run: cargo fmt --all -- --check

.github/workflows/test_web.yml

@@ -59,6 +59,7 @@ jobs:
         uses: Swatinem/rust-cache@v2
         with:
           shared-key: "web"
+          save-if: ${{ github.ref == 'refs/heads/master' }}
 
       # wasm-bindgen-cli version must match wasm-bindgen crate version.
       # Be sure to update in release_nightly.yml, web/Cargo.toml and web/README.md.