[rubygems/rubygems] Add credentials file path to "gem env".

There are 3 possible locations:

- $HOME/.gem/credentials
- $XDG_DATA_HOME/gem/credentials
- $HOME/.local/share/gem/credentials

https://github.com/rubygems/rubygems/commit/c51756b96e

lib/rubygems/commands/environment_command.rb

@@ -15,6 +15,7 @@ class Gem::Commands::EnvironmentCommand < Gem::Command
           version         display the gem format version
           remotesources   display the remote gem servers
           platform        display the supported gem platforms
+          credentials     display the path where credentials are stored
           <omitted>       display everything
     EOF
     args.gsub(/^\s+/, "")
@@ -88,6 +89,8 @@ lib/rubygems/defaults/operating_system.rb
         Gem.sources.to_a.join("\n")
       when /^platform/ then
         Gem.platforms.join(File::PATH_SEPARATOR)
+      when /^credentials/, /^creds/ then
+        Gem.configuration.credentials_path
       when nil then
         show_environment
       else
@@ -114,6 +117,8 @@ lib/rubygems/defaults/operating_system.rb
 
     out << "  - USER INSTALLATION DIRECTORY: #{Gem.user_dir}\n"
 
+    out << "  - CREDENTIALS FILE: #{Gem.configuration.credentials_path}\n"
+
     out << "  - RUBYGEMS PREFIX: #{Gem.prefix}\n" unless Gem.prefix.nil?
 
     out << "  - RUBY EXECUTABLE: #{Gem.ruby}\n"