Disable parallel built in test-bundled-gems

common.mk

@@ -1408,7 +1408,7 @@ no-test-bundled-gems:
 
 BUNDLED_GEMS =
 test-bundled-gems-run: $(PREPARE_BUNDLED_GEMS)
-	$(gnumake_recursive)$(Q) $(XRUBY) $(tooldir)/test-bundled-gems.rb $(BUNDLED_GEMS)
+	$(Q) $(XRUBY) $(tooldir)/test-bundled-gems.rb $(BUNDLED_GEMS)
 
 test-bundler-precheck: $(TEST_RUNNABLE)-test-bundler-precheck
 no-test-bundler-precheck:

tool/test-bundled-gems.rb

@@ -2,6 +2,8 @@ require 'rbconfig'
 require 'timeout'
 require 'fileutils'
 
+ENV.delete("GNUMAKEFLAGS")
+
 github_actions = ENV["GITHUB_ACTIONS"] == "true"
 
 allowed_failures = ENV['TEST_BUNDLED_GEMS_ALLOW_FAILURES'] || ''