ci: track the actions in the signing jobs by tag again (#11137)

The repository tracks actions by tag with dependabot; the signing jobs
follow the same convention.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

.github/actions/sign-image/action.yml

@@ -15,7 +15,7 @@ runs:
   using: composite
   steps:
     - name: Install cosign
-      uses: sigstore/cosign-installer@6f9f17788090df1f26f669e9d70d6ae9567deba6 # v4.1.2
+      uses: sigstore/cosign-installer@v4.1.2
 
     - name: Sign
       shell: bash