release 1.13.x: fix goreleaser (#3619)

This is using Cosign v1 to sign, so we need to experimental flag set.

Signed-off-by: Hayden B <hblauzvern@google.com>

.goreleaser.yml

@@ -4,7 +4,7 @@ env:
   - GO111MODULE=on
   - CGO_ENABLED=1
   - DOCKER_CLI_EXPERIMENTAL=enabled
-  - COSIGN_YES=true
+  - COSIGN_EXPERIMENTAL=true
   - LATEST_TAG=,latest
 
 # Prevents parallel builds from stepping on each others toes downloading modules