ci(goreleaser): only generate signature for checksums.txt (#1366)

If we only want to generate a `.sig` release artifact for
`checksums.txt`, we need to set `artifact: checksum`.
See https://goreleaser.com/customization/sign/

Author: phm07

.goreleaser.yml

@@ -125,7 +125,7 @@ checksum:
   algorithm: sha256
 
 signs:
-  - artifacts: all
+  - artifacts: checksums
     signature: ${artifact}.sig
     id: hcloud-sign
     args: