Load operating_system.rb customizations before setting up default gems

It's very common for packagers to configure gem paths in this file, for
example, `Gem.default_dir`. Also, setting up default gems requires these
paths to be set, so that we know which default gems need to be setup.

If we setup default gems before loading `operatin_system.rb`
customizations, the wrong default gems will be setup.

Unfortunately, default gems loaded by `operating_system.rb` can't be
upgraded if we do this, but it seems much of a smaller issue. I wasn't
even fully sure it was the right thing to do when I added that, and it
was not the culprit of the end user issue that led to making that
change.

.github/workflows/install-rubygems.yml

@@ -105,10 +105,5 @@ jobs:
       - name: Check we can install a Gemfile with git sources
         run: bundle init && bundle add fileutils --git https://github.com/ruby/fileutils
         shell: bash
-      - name: Check the latest available fiddle version gets activated
-        run: |
-          gem install fiddle:1.1.0
-          ruby -rfiddle -e "exit Fiddle::VERSION == '1.1.0'"
-        if: matrix.ruby.name != 'jruby-9.3'
 
     timeout-minutes: 10

lib/rubygems.rb

@@ -1323,22 +1323,6 @@ def default_gem_load_paths
 require_relative 'rubygems/specification'
 
 # REFACTOR: This should be pulled out into some kind of hacks file.
-begin
-  ##
-  # Defaults the Ruby implementation wants to provide for RubyGems
-
-  require "rubygems/defaults/#{RUBY_ENGINE}"
-rescue LoadError
-end
-
-##
-# Loads the default specs.
-Gem::Specification.load_defaults
-
-require_relative 'rubygems/core_ext/kernel_gem'
-require_relative 'rubygems/core_ext/kernel_require'
-require_relative 'rubygems/core_ext/kernel_warn'
-
 begin
   ##
   # Defaults the operating system (or packager) wants to provide for RubyGems.
@@ -1354,3 +1338,19 @@ def default_gem_load_paths
     "the problem and ask for help."
   raise e.class, msg
 end
+
+begin
+  ##
+  # Defaults the Ruby implementation wants to provide for RubyGems
+
+  require "rubygems/defaults/#{RUBY_ENGINE}"
+rescue LoadError
+end
+
+##
+# Loads the default specs.
+Gem::Specification.load_defaults
+
+require_relative 'rubygems/core_ext/kernel_gem'
+require_relative 'rubygems/core_ext/kernel_require'
+require_relative 'rubygems/core_ext/kernel_warn'

test/rubygems/test_rubygems.rb

@@ -22,6 +22,29 @@ def test_operating_system_other_exceptions
     "the problem and ask for help."
   end
 
+  def test_operating_system_customizing_default_dir
+    pend "does not apply to truffleruby" if RUBY_ENGINE == 'truffleruby'
+    pend "loads a custom defaults/jruby file that gets in the middle" if RUBY_ENGINE == 'jruby'
+
+    # On a non existing default dir, there should be no gems
+
+    path = util_install_operating_system_rb <<-RUBY
+      module Gem
+        def self.default_dir
+          File.expand_path("foo")
+        end
+      end
+    RUBY
+
+    output = Gem::Util.popen(
+      *ruby_with_rubygems_and_fake_operating_system_in_load_path(path),
+      '-e',
+      "require \"rubygems\"; puts Gem::Specification.stubs.map(&:full_name)",
+      {:err => [:child, :out]}
+    ).strip
+    assert_empty output
+  end
+
   private
 
   def util_install_operating_system_rb(content)