Build failure due to deleted `etc` files from built dependencies

[cho-m]

I was debugging ocaml bump failure (Homebrew/homebrew-core#102436) and noticed that newly built ocaml-findlib was causing failures for subsequent formulae that depended on it due to missing etc/findlib.conf.

I think the issue is due to the etc files getting cleaned up and not being restored for dependents that are also being bottled:

https://github.com/Homebrew/homebrew-test-bot/blob/c2c4be711d4dc8b5459388dda87ac3d0409b1e42/lib/tests/formulae.rb#L450-L451

Was able to test that modifying etc to pkgshare/"etc" avoided issue, but that loses the advantage of preserving user-modifications via etc directory.

[MikeMcQuaid]

Good catch @cho-m. This will need a bug fix to test-bot.