Use the `nix` command (and flakes) in the CI

Apart from a slight simplification and a bit of dogfooding, this also
make the cache behavior more predictable.
For example `nix build .` and `nix build nix/$(git rev-parse HEAD)` will
yield the exact same path, while their “intuitive” non-flake equivalents
(`nix-build` and
`nix-build https://github.com/nixos/nix/archives/$(git rev-parse HEAD).tar.gz`)
don’t.

This was a pain for example in https://github.com/NixOS/nix/pull/5059

Also, the `bar-with-logs` log format is imho nicer (even in an
non-interactive context) because prefixing each log line with the name
of the derivation that produced it makes it much easier to follow what’s
going on.

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