Tweaks to freeze bundler CI

Currently some specs are broken because `rspec-mocks-3.10.3` is used,
which has some breaking changes, apparently.

This change makes ruby-core install the same gems installed upstream for
running bundle specs, so that things never break with 3rd party
releases.

common.mk

@@ -1412,8 +1412,7 @@ yes-test-bundler-precheck: main
 no-test-bundler-prepare: no-test-bundler-precheck
 yes-test-bundler-prepare: yes-test-bundler-precheck
 	$(ACTIONS_GROUP)
-	$(XRUBY) -C "$(srcdir)" bin/gem install --no-document \
-		--install-dir .bundle --conservative "rspec:~> 3.8" "rake:~> 13.0" "parallel_tests:~> 2.29"
+	$(XRUBY) -C $(srcdir) -Ilib -e 'ENV["GEM_HOME"] = File.expand_path(".bundle"); ENV["BUNDLE_APP_CONFIG"] = File.expand_path(".bundle"); ENV["BUNDLE_PATH__SYSTEM"] = "true"; ENV["BUNDLE_WITHOUT"] = "lint doc"; load "spec/bundler/support/bundle.rb"' -- install --gemfile=tool/bundler/dev_gems.rb
 	$(ACTIONS_ENDGROUP)
 
 RSPECOPTS =