[rubygems/rubygems] Make sure --no-user-install is respected for auto user installation

The `options[:user_install]` might have three states:
* `true`: `--user-install`
* `false`: `--no-user-install` and
* `nil`: option was not specified

However, this had not been respected previously and the `false` and `nil`
were treated the same. This could lead to auto user installation despite
`--no-user-install` being specified on the command line.

Fixes https://github.com/rubygems/rubygems/pull/7237

9281545474

lib/rubygems/installer.rb

@@ -682,7 +682,7 @@ class Gem::Installer
       if @build_root.nil?
         if options[:user_install]
           @gem_home = Gem.user_dir
-        elsif !ENV.key?("GEM_HOME") && (File.exist?(Gem.dir) && !File.writable?(Gem.dir))
+        elsif options[:user_install].nil? && !ENV.key?("GEM_HOME") && (File.exist?(Gem.dir) && !File.writable?(Gem.dir))
           say "Defaulting to user installation because default installation directory (#{Gem.dir}) is not writable."
           @gem_home = Gem.user_dir
         end

test/rubygems/test_gem_installer.rb

@@ -1995,6 +1995,27 @@ end
     ENV["GEM_HOME"] = orig_gem_home
   end
 
+  def test_process_options_does_not_fallback_to_user_install_when_gem_home_not_writable_and_no_user_install
+    if Process.uid.zero?
+      pend("skipped in root privilege")
+      return
+    end
+
+    orig_gem_home = ENV.delete("GEM_HOME")
+
+    @gem = setup_base_gem
+
+    FileUtils.chmod 0o000, @gemhome
+
+    installer = use_ui(@ui) { Gem::Installer.at @gem, user_install: false }
+
+    assert_equal @gemhome, installer.gem_home
+    assert_empty @ui.output.strip
+  ensure
+    FileUtils.chmod 0o755, @gemhome
+    ENV["GEM_HOME"] = orig_gem_home
+  end
+
   def test_shebang_arguments
     load_relative "no" do
       installer = setup_base_installer