Use Nix for GitLab pipelines
This MR changes .gitlab-ci.yml to use nix instead of opam/rustup.
Furthermore, the .gitlab-ci.yml has been completely rewritten to use artifacts/cache and take advantage of parallelism using jobs.
This MR changes .gitlab-ci.yml to use nix instead of opam/rustup.
Furthermore, the .gitlab-ci.yml has been completely rewritten to use artifacts/cache and take advantage of parallelism using jobs.