auto-pr: fix setup-homebrew, env usage

[woodruffw]

No description provided.

[request-info]

Please provide a better issue/pull request title and/or description!

[woodruffw]

Hmm, this didn't do the trick:

==> certbot: marking as revision 2
/usr/local/Homebrew/Library/Homebrew/utils/ast.rb:150:in `replace_stanza': Could not find 'revision' stanza! (RuntimeError)
	from generate-prs.rb:106:in `block in <main>'
	from generate-prs.rb:59:in `each'
	from generate-prs.rb:59:in `<main>'
Error: Process completed with exit code 1.

https://github.com/Homebrew/brew-pip-audit/actions/runs/5489503701/jobs/10003741749