om ci: Run nix flake check

[srid]

Not all checks are done by building a derivation (what nixci build does currently).

For eg., nix flake check tests for unknown properties in templates.* and errors out:

https://github.com/NixOS/nix/blob/babfd0cd99012df9d645d38e47fa31d049bf01d8/src/nix/flake.cc#L550-L554

The above will not be caught by nixci build.

Do we care to support this niche case, though?

--

More generally, nix flake check does a bunch of "Evaluation checks":

[srid]

I wonder if we can decouple these checks through flake-schemas.