Fix Makefile syntax in justfile

Author: stefanvanburen

justfile

@@ -69,4 +69,4 @@ generate: generate-conformance generate-example generate-test format
 
 # Used in CI to verify that `just generate` doesn't produce a diff
 checkgenerate: generate
-	test -z "$$(git status --porcelain | tee /dev/stderr)"
+	test -z "$(git status --porcelain | tee /dev/stderr)"