treewide: assemble all `fetchurlBoot` uses in overrides to `fetchurl` itself

The only outside-curl uses of `fetchurlBoot` left are `stdenv`
and `apple-source-releases`. The latter one can probably be removed
too, but I can't test it.

Pros:

- Aggregates all behind-the-scenes insanity in a single place.

Cons:

- At the cost of 10 more derivations (but 0 new outpaths).