Cache always misses for tags, so save space and do not save back

.github/workflows/build.yml

@@ -42,6 +42,7 @@ jobs:
     - run: npm ci
     - name: Cache PlatformIO
       uses: actions/cache@v4
+      if: !startsWith(github.ref, 'refs/tags/')
       with:
         path: |
               ~/.platformio/.cache