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).

pkgs/development/libraries/libssh2/default.nix

@@ -1,9 +1,9 @@
-{ stdenv, fetchurlBoot, openssl, zlib, windows }:
+{ stdenv, fetchurl, openssl, zlib, windows }:
 
 stdenv.mkDerivation rec {
   name = "libssh2-1.8.0";
 
-  src = fetchurlBoot {
+  src = fetchurl {
     url = "${meta.homepage}/download/${name}.tar.gz";
     sha256 = "1m3n8spv79qhjq4yi0wgly5s5rc8783jb1pyra9bkx1md0plxwrr";
   };