[nix] add stack, because it's needed for tests

4 jobs for 341-dev-websockets in 45 minutes and 39 seconds (queued for 3 minutes and 13 seconds)
Status Job ID Name Coverage
  Cabal
passed #14249
cabal

00:11:40

 
  Stack
passed #14250
stack

00:05:04

 
  Bench
manual #14251
allowed to fail manual
bench
 
  Test
failed #14252
test

00:28:53

 
Name Stage Failure
failed
test Test
    module ‘Test.API.Notifications’ cannot be found locally

<no location info>: error:
module ‘Test.Instances’ cannot be found locally
Error: cabal: Failed to build test:garg-test-hspec from gargantext-0.0.7.1.14.
Failed to build test:garg-test-tasty from gargantext-0.0.7.1.14.

Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1