This should help with debugging `Gem::LoadError`s (e.g. for
https://github.com/Homebrew/homebrew-test-bot/pull/111).

Don't add these unconditionally or with Bundler to speed things up a
bit. Also, to speed things up further cache these arguments so we don't
have to do the same generation every time.

The `brew test` tests were failing as they were unable to include
`test/unit/assertions`. This is because it's a gem and we were setting
the `GEM_HOME` so system gems were being ignored.

While I was there and examining the `$LOAD_PATH`: reduce the number of
things we add there by using `Bundler.with_clean_env` and only adding
`HOMEBREW_LIBRARY_PATH` in the `$LOAD_PATH` if it isn't already there
(which it always is [and has to be] for integration tests).

This also seems to have the side effect of speeding up integration
tests from 1m26s to 1m8s on my machine.

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

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.