fix: revert cosign-installer to v3 — v4 major tag does not exist yet

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mikkeldamsgaard

.github/workflows/release.yml

@@ -69,7 +69,7 @@ jobs:
           file bin/initium-amd64 bin/initium-arm64
           echo "amd64 size: $(du -h bin/initium-amd64 | cut -f1)"
           echo "arm64 size: $(du -h bin/initium-arm64 | cut -f1)"
-      - uses: sigstore/cosign-installer@v4
+      - uses: sigstore/cosign-installer@v3
         if: env.DRY_RUN == 'false'
       - uses: docker/setup-buildx-action@v4
       - uses: docker/login-action@v4