Fix type and style errors

Library/Homebrew/attestation.rb

@@ -75,6 +75,7 @@ module Homebrew
       # NOTE: We set HOMEBREW_NO_VERIFY_ATTESTATIONS when installing `gh` itself,
       #       to prevent a cycle during bootstrapping. This can eventually be resolved
       #       by vendoring a pure-Ruby Sigstore verifier client.
+      @gh_executable ||= T.let(nil, T.nilable(Pathname))
       return @gh_executable if @gh_executable.present?
 
       with_env(HOMEBREW_NO_VERIFY_ATTESTATIONS: "1") do
@@ -88,7 +89,7 @@ module Homebrew
         end
       end
 
-      @gh_executable
+      T.must(@gh_executable)
     end
 
     # Verifies the given bottle against a cryptographic attestation of build provenance.

Library/Homebrew/cmd/install.rb

@@ -265,7 +265,7 @@ module Homebrew
 
         if Homebrew::Attestation.enabled?
           if formulae.include?(Formula["gh"])
-            formulae.unshift(formulae.delete(Formula["gh"]))
+            formulae.unshift(T.must(formulae.delete(Formula["gh"])))
           else
             Homebrew::Attestation.ensure_gh_installed!
           end