- use e.g. `$HOMEBREW_*` for cases where only the environment variable
is the entire backtick-quoted string
- use e.g. `${HOMEBREW_*}` for cases where the environment variable is
part of a backtick-quoted string to make clear what parts are variable
and what parts are not
- use `export HOMEBREW_*=...` for cases where we're talking about
setting the environment variable (because it likely needs to be
exported to work how they want)
Inspired by https://github.com/Homebrew/homebrew-bundle/pull/1579 making
similar changes for Homebrew/homebrew-bundle.
