ci(fixNixBuild): Fix nix test on CI

[Gau-thier]

Closes #16

Depends on

• [ ] PR #5

[Gau-thier]

• Nix CI had been fixed within #5 (see nix/generic.nix file)
• opam 4.13.x CI is not "broken". There is a format trouble

I decided to close this PR and rework the trouble in a different one, to be sure to get rid of any dumb manipulation I have done here.