[rubygems/rubygems] Fix spec to test the right thing on bundler 3

Bundler 3 installs by default to `.bundle`. That means that, because the
`bar` gem was not previously available at this location but as a system
gem, the initial `bundle install` was silently failing. As a
consequence, the spec was not testing the exact scenario it meant to
test.

https://github.com/rubygems/rubygems/commit/202399521c

spec/bundler/install/gemfile/path_spec.rb

@@ -610,7 +610,7 @@ RSpec.describe "bundle install with explicit source paths" do
 
     it "switches the source when the gem existed in rubygems and the path was already being used for another gem" do
       build_lib "foo", "1.0", :path => lib_path("foo")
-      build_gem "bar", "1.0", :to_system => true do |s|
+      build_gem "bar", "1.0", :to_bundle => true do |s|
         s.write "lib/bar.rb", "raise 'fail'"
       end