chore(deps): update sigstore/cosign-installer digest to b18d21a (#1087)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| sigstore/cosign-installer | action | digest | `9614fae` -> `b18d21a` |

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.github/workflows/release-please.yaml

@@ -117,7 +117,7 @@ jobs:
             COMMIT=${{ github.sha }}
             DATE=${{ needs.release-please.outputs.date }}
       - name: Install Cosign
-        uses: sigstore/cosign-installer@9614fae9e5c5eddabb09f90a270fcb487c9f7149
+        uses: sigstore/cosign-installer@b18d21aaa20ef40c4e681202a598b7591d5be577
 
       - name: Sign the image
         run: |