ci: pin sigstore/cosign-installer to v4.1.2

The action does not publish a moving v4 major tag; the workflow failed to
resolve sigstore/cosign-installer@v4. Pin to the latest full version.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mtzanidakis

.github/workflows/build.yml

@@ -25,7 +25,7 @@ jobs:
       - uses: actions/checkout@v6
 
       - name: Install cosign
-        uses: sigstore/cosign-installer@v4
+        uses: sigstore/cosign-installer@v4.1.2
         with:
           cosign-release: 'v3.0.6'