extend/kernel: add sigs to opoo/onoe/ofail

Add these and correctly pass through a string to
`GitHub::Actions.puts_annotation_if_env_set`.

Also, fix some call sites to not rely on the `void` return.

Library/Homebrew/cmd/link.rb

@@ -45,6 +45,8 @@ module Homebrew
               To install, run:
                 brew install --HEAD #{name}
             EOS
+
+            nil
           end
         else
           args.named.to_latest_kegs

Library/Homebrew/dev-cmd/bump-unversioned-casks.rb

@@ -125,6 +125,8 @@ module Homebrew
             end
           rescue => e
             onoe e
+
+            nil
           end
 
           if sha256.present? && last_sha256 != sha256
@@ -134,6 +136,8 @@ module Homebrew
               end
             rescue Timeout::Error
               onoe "Timed out guessing version for cask '#{cask}'."
+
+              nil
             end
 
             if version

Library/Homebrew/extend/kernel.rb

@@ -61,26 +61,29 @@ module Kernel
   # Print a warning message.
   #
   # @api public
+  sig { params(message: T.any(String, Exception)).void }
   def opoo(message)
     Tty.with($stderr) do |stderr|
       stderr.puts Formatter.warning(message, label: "Warning")
-      GitHub::Actions.puts_annotation_if_env_set(:warning, message)
+      GitHub::Actions.puts_annotation_if_env_set(:warning, message.to_s)
     end
   end
 
   # Print an error message.
   #
   # @api public
+  sig { params(message: T.any(String, Exception)).void }
   def onoe(message)
     Tty.with($stderr) do |stderr|
       stderr.puts Formatter.error(message, label: "Error")
-      GitHub::Actions.puts_annotation_if_env_set(:error, message)
+      GitHub::Actions.puts_annotation_if_env_set(:error, message.to_s)
     end
   end
 
   # Print an error message and fail at the end of the program.
   #
   # @api public
+  sig { params(error: T.any(String, Exception)).void }
   def ofail(error)
     onoe error
     Homebrew.failed = true