`brew bundle exec` behaves correctly only after doing `brew bundle install`.
Running `brew bundle check` can be slow, so let's add a `--check` flag
to `brew bundle exec` which will also run `brew bundle check` before
`brew bundle exec` to ensure that the `Brewfile` has been installed
before proceeding.
- if HOMEBREW_TEMP is not writable, use the default temp directory
- when running `brew bundle exec`, strip various temporary directories
from the environment if they are not writable
- Rather than maintaining a list of Homebrew environment variables to
delete, delete all Homebrew (and Portable Ruby) internal variables
that won't be used by other tools.
- When exporting variables, only export variables that have changed.
- When exporting PATH-like variables, ensure the PATH is appending to
the existing path rather than replacing it and ensure we only include
newly added paths.
- Sort output by key so it's more readable.
- Skip exporting empty values because shell scripts treat them as unset.
- Skip exporting non-Homebrew things that were already set in the old
environment to avoid massive duplication and higher chances of e.g.
bad escapes breaking things.
