Fix goreleaser snapshot signing creating missing bundle file

When running in snapshot mode, the signing step skips cosign but does
not create the expected .bundle file. The checksum step then fails
because it cannot find the signature artifact. Touch the file so the
pipeline can continue.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

.goreleaser.yaml

@@ -142,6 +142,7 @@ signs:
       - |
         if [ "{{ .IsSnapshot }}" = "true" ]; then
           echo "Skipping signing (snapshot: {{ .IsSnapshot }})"
+          touch "${signature}"
         else
           cosign sign-blob --bundle="${signature}" "${artifact}" --yes
         fi