Remove `testbottest` formula (this should have lived in the `brew
test-bot` repository from the outset) and make various other changes to
fix the Linux build here.

Support GPG (signed data) container in Homebrew Cask

This reverts commit 3e4547f52e7ebec633f8bfefc8a396d944edf908, reversing
changes made to 6edf9382bcc1240ad6f97c8b752cfe56cef9965d.

This reverts commit 2372872974d1049c2beafe7dedb7f8f882502058, reversing
changes made to 3e4547f52e7ebec633f8bfefc8a396d944edf908.

If people have `HOMEBREW_RUBY_PATH` set then things explode in a rather
confusing fashion. Instead, run `bundle` for them with the arguments
that they'd want.

Also, move `macho` requires into the module itself; it's a pain having
to do everything for Bundler before requiring `pathname` which is a core
Ruby class.

Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.

Hide these tokens to avoid malicious subprocesses e.g. sending them
over the network. Also, support using these tokens with environment
filtering and clear `HOMEBREW_PATH` from subprocesses to stop them
sniffing it. Finally, use `HOMEBREW_PATH` to detect Homebrew’s user’s
PATH for e.g. `brew doctor` etc.