The old stdenv didn't work, and was also impure. The new one works, and
is pure. Presently, the bootstrap tools are cross compiled into one small
nar and one large tar, which is then unpacked, patched, and split into
smaller derivations. Efforts were made to make the boot process as short
as possible - there are only two clangs built, and as many packages are
propagated between stages as possible while leaving the bootstrap tools
out of the final stdenv's closure.
Since the list only gates the platforms the nixpkgs flake exposes
packages to build on, the `hydra` label made little sense. It was also
only used for this purpose, so the `tier*` attributes were largely
unnecessary.
To reflect the intention more accurately, we expose
`lib.systems.flakeExposed` and use it to gate flake.nix's system list.
