utils/github: fix frozen string usage.

See https://discourse.brew.sh/t/error-cant-modify-frozen-string/4691/4

Library/Homebrew/utils/github.rb

@@ -47,7 +47,7 @@ module GitHub
   class AuthenticationFailedError < Error
     def initialize(github_message)
       @github_message = github_message
-      message = "GitHub #{github_message}:"
+      message += "GitHub #{github_message}:"
       if ENV["HOMEBREW_GITHUB_API_TOKEN"]
         message << <<~EOS
           HOMEBREW_GITHUB_API_TOKEN may be invalid or expired; check:
@@ -63,7 +63,7 @@ module GitHub
           #{Utils::Shell.set_variable_in_profile("HOMEBREW_GITHUB_API_TOKEN", "your_token_here")}
         EOS
       end
-      super message
+      super message.freeze
     end
   end