this replaces niv with nix flakes, attempting to preserve the old
structure as much as possible. Notable caveats:
- I'm not sure if flake inputs expose version information anywhere, so
the version in pkgs/mattermost/default.nix is now hardcoded.
Confusingly, this appears to trigger a rebuild. Maybe I've missed something.
- a lot of the old-style host.nix & deploy.nix machinery in nix-hexchen
does not work with flakes, and their newer replacements are not exposed
by upstream; I've put basic imitations of the relevant parts in this repo
- (in particular, directories in hosts/ won't become deployable configs
automatically)
- parts of the code are now probably more complicated than they'd have to be
- old variables names were preserved; confusingly, this means the flake
inputs are still called "sources"
