prefer `pass` backend if it is available

Signed-off-by: Tycho Andersen <tycho@docker.com>

cli/config/credentials/default_store_linux.go

@@ -1,5 +1,13 @@
 package credentials
 
+import (
+	"github.com/docker/docker-credential-helpers/pass"
+)
+
 func defaultCredentialsStore() string {
+	if pass.PassInitialized {
+		return "pass"
+	}
+
 	return "secretservice"
 }