fix(ci/goreleaser): pin cosign version to v2.6.1 (#2346)

cf.

- goreleaser/goreleaser#6194
- https://github.com/future-architect/vuls/actions/runs/19059752068/job/54440781875

Author: shino

.github/workflows/goreleaser.yml

@@ -16,6 +16,8 @@ jobs:
     steps:
       - name: Cosign install
         uses: sigstore/cosign-installer@faadad0cce49287aee09b3a48701e75088a2c6ad
+        with:
+          cosign-release: "v2.6.1"
       - name: Maximize build space
         uses: easimon/maximize-build-space@fc881a613ad2a34aca9c9624518214ebc21dfc0c
         with: