Update sigstore/cosign-installer action to v4 (#996)

| datasource  | package                   | from   | to     |
| ----------- | ------------------------- | ------ | ------ |
| github-tags | sigstore/cosign-installer | v3.8.2 | v4.0.0 |

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

@@ -91,7 +91,7 @@ jobs:
         if: github.ref_type == 'tag'
 
       - name: Install Cosign
-        uses: sigstore/cosign-installer@3454372f43399081ed03b604cb2d021dabca52bb # v3.8.2
+        uses: sigstore/cosign-installer@faadad0cce49287aee09b3a48701e75088a2c6ad # v4.0.0
         if: github.ref_type == 'tag'
 
       - name: Setup Snapcraft