Explicitly handle `dont*` and friends not being defined.

Same justification as previous commit.

Go beyond the obvious setup hooks now, with a bit of sed, with a skipped case:

 - cc-wrapper's `dontlink`, because it already is handled.

Also, in nix files escaping was manually added.

EMP

If these aren't defined, the stdenv defaults are used in the `*Phase`
case, or no extra phases are done, in the `*Phases` case.

Before, we very carefully unapplied and reapplied `set -u` so the rest
of Nixpkgs could continue to not fail on undefined variables. Let's rip
off the band-aid.

My earlier sed missed this because it already had `{..}`.

I don't want to just rebase this away because the original commit is
also in #72074.

treewide: `set -u` everywhere