bump-*-pr: handle HOMEBREW_NO_GITHUB_API=1 being set

We early return here
https://github.com/homebrew/brew/blob/92a4311868322188478d7a90511ec0e8e6b0d7df/Library/Homebrew/utils/github/api.rb#L220
, but don't then handle that through the stack.

Repro:

```console
❯ HOMEBREW_NO_AUTO_UPDATE=1 HOMEBREW_NO_GITHUB_API=1 brew bump-formula-pr --write-only --version 1.2.3 --no-audit jq
Error: undefined method `[]' for nil:NilClass
Do not report this issue until you've run `brew update` and tried again.
Warning: Removed Sorbet lines from backtrace!
/opt/homebrew/Library/Homebrew/utils/github.rb:565:in `block in fetch_pull_requests'
/opt/homebrew/Library/Homebrew/utils/github/api.rb:334:in `paginate_graphql'
/opt/homebrew/Library/Homebrew/utils/github.rb:564:in `fetch_pull_requests'
/opt/homebrew/Library/Homebrew/utils/github.rb:628:in `check_for_duplicate_pull_requests'
/opt/homebrew/Library/Homebrew/dev-cmd/bump-formula-pr.rb:456:in `check_open_pull_requests'
/opt/homebrew/Library/Homebrew/dev-cmd/bump-formula-pr.rb:135:in `run'
/opt/homebrew/Library/Homebrew/brew.rb:89:in `<main>'
Rerun with `--verbose` to see the original backtrace
```

Library/Homebrew/utils/github.rb

@@ -521,6 +521,8 @@ def self.pull_request_title_regex(name, version = nil)
  .returns(T::Array[T::Hash[String, T.untyped]])
  }
  def self.fetch_pull_requests(name, tap_remote_repo, state: nil, version: nil)
+ return [] if Homebrew::EnvConfig.no_github_api?
+
  regex = pull_request_title_regex(name, version)
  query = "is:pr #{name} #{version}".strip