debug: add lookup-only step to diagnose cache backend

.gitea/actions/rust-cache/action.yml

@@ -27,6 +27,20 @@ runs:
         echo "hash=$HASH" >> $GITHUB_OUTPUT
         echo "fingerprint=$HASH" >> $GITHUB_ENV
 
+    - name: Lookup cache (debug)
+      id: cache-lookup
+      uses: https://gitea.com/actions/cache@v3
+      with:
+        path: ~/.cargo/registry
+        key: rust-linux-${{ steps.cache-key.outputs.hash }}
+        lookup-only: true
+
+    - name: Show lookup result
+      shell: bash
+      run: |
+        echo "cache-hit (lookup): ${{ steps.cache-lookup.outputs.cache-hit }}"
+        echo "key used: rust-linux-${{ steps.cache-key.outputs.hash }}"
+
     - name: Cache cargo + rustup
       id: cache
       uses: https://gitea.com/actions/cache@v3