replace nix-shell with Nix Flake

[peterbecich]

This flake.nix provides the same resources as the deleted Nix files with less code to maintain, plus other tools helpfully provided by https://github.com/srid/haskell-flake.

This commit does not fix/update the docs generation via Nix; this remains TODO.

The flake.nix will build bundles for each Beam library, just as the old Nix configuration did:

$ nix flake show
...
    └───x86_64-linux
        ├───all: package 'all'
        ├───beam-core: package 'beam-core-0.10.3.1'
        ├───beam-migrate: package 'beam-migrate-0.5.3.1'
        ├───beam-postgres: package 'beam-postgres-0.5.4.1'
        ├───beam-sqlite: package 'beam-sqlite-0.5.3.1'
        ├───default: package 'all'
        └───pagila: package 'pagila-0.1.0.0'

nix build or nix build .#beam-... will build these.

Beam has no executables so there is nothing to demonstrate by nix run .#...

I need more time to understand the docs generation, which apparently requires a Beam executable. This may have been the beam-migrate executable, which no longer exists. Please review this; I will try to return the the docs as a follow-up item.

[peterbecich]

Tracking the .envrc in version control is debated; personally I am in favor of it

[kmicklas]

Personally I would prefer not to commit to this, since a lot more people have direnv than Nix, and Nix isn't required to easily develop on this repo.

[peterbecich]

check the flake.nix and its outputs are not broken

[peterbecich]

this file is made dead code by this PR. I want to follow-up on this later.

[LaurentRDC]

I don't use Nix so I will leave the review of this PR to someone else

[peterbecich]

@kmicklas please review. I will fix docs/default.nix later.

[kmicklas]

Thanks for getting the CI to work with this!

Personally I am a flakes conscientious objector so unfortunately I cannot really review if the flake setup looks good. However from general Nix usage this all looks very nice.

[kmicklas]

Personally I would prefer not to commit to this, since a lot more people have direnv than Nix, and Nix isn't required to easily develop on this repo.

[kmicklas]

Is hp haskellPackages? If so I would prefer it to be more explicit for readability here.