ci: Install cosign to sign assets

Author: armsnyder

.github/workflows/assets.yaml

@@ -10,6 +10,14 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+      - name: Install Cosign
+        uses: sigstore/cosign-installer@v3
+        with:
+          version: v2.0.0
       - name: Create app token
         uses: actions/create-github-app-token@v2
         id: app-token