Add --source option to gem exec command

Allows specifying a custom gem source when running `gem exec`, matching
the existing --source option on gem install.

Co-Authored-By: akshay-birajdar-zendesk <akshay.birajdar@zendesk.com>

Author: the-spectator

lib/rubygems/commands/exec_command.rb

@@ -3,17 +3,20 @@
 require_relative "../command"
 require_relative "../dependency_installer"
 require_relative "../gem_runner"
+require_relative "../local_remote_options"
 require_relative "../package"
 require_relative "../version_option"
 
 class Gem::Commands::ExecCommand < Gem::Command
   include Gem::VersionOption
+  include Gem::LocalRemoteOptions
 
   def initialize
     super "exec", "Run a command from a gem", {
       version: Gem::Requirement.default,
     }
 
+    add_source_option
     add_version_option
     add_prerelease_option "to be installed"
 

test/rubygems/test_gem_commands_exec_command.rb

@@ -143,6 +143,28 @@ def test_gem_with_executable
     end
   end
 
+  def test_gem_with_executable_from_custom_source
+    spec_fetcher "http://other-gems.example.com/" do |fetcher|
+      fetcher.gem "a", 2 do |s|
+        s.executables = %w[a]
+        s.files = %w[bin/a lib/a.rb]
+
+        write_file File.join(*%W[gems #{s.original_name} bin a]) do |f|
+          f << "Gem.ui.say #{s.original_name.dump}"
+        end
+      end
+    end
+
+    util_clear_gems
+
+    use_ui @ui do
+      @cmd.invoke "--source", "http://other-gems.example.com", "a:2"
+      assert_equal "a-2\n", @ui.output
+    end
+
+    assert_include Gem.sources.to_a, "http://other-gems.example.com/"
+  end
+
   def test_gem_with_platforms
     spec_fetcher do |fetcher|
       fetcher.download "a", 2 do |s|