Setting language: nix seems to override my dist: xenial setting. See https://travis-ci.org/vaibhavsagar/ghc-dev-ci/jobs/548290867.
It seems to me that the failure is coming from your script’s failure to clone ghc from their GitLab installation.
I don’t know what happened, but the latest build gets past that spot. https://travis-ci.org/vaibhavsagar/ghc-dev-ci/builds/548330307
The script’s failure is because it’s running on Ubuntu 14.04. The latest build succeeded because I installed Nix manually instead of using language: nix.