Avoid polluting the repo with temporary `channels.{nix,json,toml}`
files. Write them to a temporary working directory instead.
(cherry picked from commit bc997a2409)
Flake registries and include paths are not the same thing.
`<nixpkgs>` looks for nixpkgs on the include path, set by `NIX_PATH` and
`--include`. However `--inputs-from` adds inputs to the flake registry,
not the include path.
(cherry picked from commit 2369b7fc4f)
