Update Library/Homebrew/utils/github/api.rb

Co-authored-by: Rylan Polster <rslpolster@gmail.com>

Author: MikeMcQuaid

Library/Homebrew/utils/github/api.rb

@@ -198,7 +198,7 @@ def self.keychain_username_password
 
     sig { returns(T.nilable(String)) }
     def self.credentials
-      @credentials = T.let(nil, T.nilable(String))
+      @credentials ||= T.let(nil, T.nilable(String))
       @credentials ||= Homebrew::EnvConfig.github_api_token.presence
       @credentials ||= github_cli_token
       @credentials ||= keychain_username_password