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:

homebrew-test-bot/lib/tests/formulae.rb

Lines 450 to 451 in c2c4be7

	ensure
	cleanup_bottle_etc_var(formula) if args.cleanup?

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.