utils/github: Fix 'uninitialized constant Utils::Shell'

- I tried `brew pr-publish` on Linux (as we don't have an auto-merge
  Action over there) and it failed with the following error:

```
Error: uninitialized constant Utils::Shell
Please report this issue:
  https://docs.brew.sh/Troubleshooting
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/github.rb:69:in `initialize'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/github.rb:278:in `exception'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/github.rb:278:in `raise'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/github.rb:278:in `raise_api_error'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/github.rb:228:in `open_api'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/github.rb:464:in `workflow_dispatch_event'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/pr-publish.rb:43:in `block in pr_publish'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/pr-publish.rb:33:in `each'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/pr-publish.rb:33:in `pr_publish'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:119:in `<main>'
```