update nix-shell sources; GitHub Actions for nix-shell

[peterbecich]

• update nix-shell sources
• use niv (https://github.com/nmattia/niv)
• GitHub Actions to check nix-shell

---

nix-shell builds the project successfully in GitHub Actions and locally:

 nix-shell --pure --run "cabal update && cabal build all --enable-tests"    
...
Downloading the latest package list from hackage.haskell.org
Updated package list of hackage.haskell.org to the index-state 2022-02-13T21:11:53Z
To revert to previous state run:
    cabal v2-update 'hackage.haskell.org,2022-02-13T18:44:13Z'
Resolving dependencies...
Up to date

[gbaz]

Why is nix/sources.nix checked in here? Its a big autogenerated file -- I confess I'm not super familiar with this stuff, but its a bit weird to me that such a file would need to be checked in.

[peterbecich]

I can delete the Niv https://github.com/nmattia/niv stuff and update the Nix channel manually if you prefer.

A.f.a.i.k. Niv currently requires the nix/sources.nix file. It was generated by niv init. This is an open issue about getting rid of this file: nmattia/niv#272

In this PR, shell.nix imports nix/sources.nix and fails without it:

nix-shell --pure
error: getting status of '.../nix/sources.nix': No such file or directory
(use '--show-trace' to show detailed location information)

I could inline sources.nix into shell.nix somehow but this would go against the Niv documentation. Probably better to just not use Niv in this case.

---

Regardless of Niv, the Nix channel is updated and brotli is provided now; readme updated: https://github.com/haskell/hackage-server/pull/1002/files#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5L24

[peterbecich]

I see the "Test nix-shell" GitHub Action takes a long time. I think it does not cache. More discussion about this: actions/cache#749 (comment)
I could try the solution discussed there, or wait for this to possibly be incorporated into https://github.com/cachix/install-nix-action

[gbaz]

if you hold off on the niv stuff i'll merge the basic fixes.

[peterbecich]

NixOS/nixpkgs@1d370bd

[peterbecich]

@gbaz , done, sorry for the delay