CI: build: limit cache save/delete only on push events

Limit ccache cache save/delete only on push events. Saving ccache
cache for pull request will result in bloat and refreshing ccache is not
possible due to security measure on enforcing read permission on
pull_request events.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

.github/workflows/build.yml

@@ -498,7 +498,7 @@ jobs:
           path: "openwrt/logs"
 
       - name: Delete already present ccache cache
-        if: steps.restore-ccache-cache.outputs.cache-hit == 'true' && inputs.use_ccache_cache == true
+        if: steps.restore-ccache-cache.outputs.cache-hit == 'true' && inputs.use_ccache_cache == true  && github.event_name == 'push'
         uses: octokit/request-action@v2.x
         with:
           route: DELETE /repos/{repository}/actions/caches?key={key}
@@ -508,7 +508,7 @@ jobs:
           INPUT_KEY: ${{ steps.restore-ccache-cache.outputs.cache-primary-key }}
 
       - name: Save ccache cache
-        if: inputs.use_ccache_cache == true
+        if: inputs.use_ccache_cache == true && github.event_name == 'push'
         uses: actions/cache/save@v3
         with:
           path: openwrt/.ccache