Fix `brew pull` for taps

Currently `brew pull` fails for some taps because they are stored in a `username-tapname` directory, but the pull command tries to chdir to `homebrew-tapname` (I assume because the core repo and the taps at Homebrew/* are the ones you usually use the pull command for).

To fix this I simply added an extra capture for the username. Only `brew-pull` and `brew-test-bot` use `HOMEBREW_PULL_URL_REGEX` so I made sure to fix the references to the MatchData in both (test-bot didn't need any changes).