fix(goreleaser): Use cosign experimental mode to enable old behaviour (#2508)

Reviewed-by: Alexander Jung <alex@unikraft.com>
Approved-by: Alexander Jung <alex@unikraft.com>

Author: nderjung

.goreleaser-stable.yaml

@@ -68,6 +68,8 @@ signs:
       - "${artifact}"
       - "--yes"
     artifacts: all
+    env:
+     - COSIGN_EXPERIMENTAL=1
 
 nfpms:
   - vendor: Unikraft