Update sigstore/cosign-installer action to v3.8.0 (#976)

| datasource  | package                   | from   | to     |
| ----------- | ------------------------- | ------ | ------ |
| github-tags | sigstore/cosign-installer | v3.7.0 | v3.8.0 |

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -159,7 +159,7 @@ jobs:
         if: github.ref_type == 'tag'
 
       - name: Install Cosign
-        uses: sigstore/cosign-installer@dc72c7d5c4d10cd6bcb8cf6e3fd625a9e5e537da # v3.7.0
+        uses: sigstore/cosign-installer@c56c2d3e59e4281cc41dea2217323ba5694b171e # v3.8.0
         if: github.ref_type == 'tag'
 
       - name: Setup Snapcraft