Add info on env variables for Formula Cookbook

[davidjb]

This explains the levels of environment variable filtering present in Homebrew, explaining how and why variables without a HOMEBREW_ prefix or those which contain TOKEN etc are made unavailable to a Formula.

Previously, this behaviour was unclear (eg HOMEBREW_FOO works but HOMEBREW_TOKEN didn't) so I've added this documentation with references explaining the process. There may be other env filtering at work, but these are the two levels that were influencing my attempts to figure out why a third party formula written some time ago was no longer able to see its environment variable.

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[MikeMcQuaid]

Great work here, thanks for the docs ❤️

[davidjb]

@MikeMcQuaid PR updated with the changes, plus some improvements in readability for the rest.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @davidjb!