[nix] add nanomsg to nix pkgs

Also, fix ./start to use gargantext-settings.toml
4 jobs for dev-add-nanomsg-to-nix in 39 minutes and 59 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Cabal
passed #15258
cabal

00:14:59

 
  Stack
passed #15259
stack

00:06:42

 
  Bench
manual #15260
allowed to fail manual
bench
 
  Test
passed #15261
test

00:18:17