Authentication for private GitLab repos need to use PAT

[toph-allen]

I'm pretty sure that GitLab's API does not allow authenticating with usernames and password (see here). This prevents Packrat's GitLab downloader from working.

There may be some circumstance or version I'm unaware of (an old self-hosted instance maybe?) where password auth is allowed. But we should probably switch to preferring a GITLAB_PAT variable if it exists in the environment.

[toph-allen]

We verified this against private GitLab repos:

• packrat::restore() can restore package sources hosted in GitLab private repos when provided a GITLAB_PAT
• packrat::restore() does not restore successfully when the incorrect GITLAB_PAT is provided. An error message is displayed instructing users to check that variable.