chore: Read Gitlab server URL from env for prime integrations

[Dschoordsch]

Description

Right now it is hardcoded, but private Gitlab instances behave like the public one, so for some Parabol instances it might make sense to point it to a different Gitlab instance.

Demo

https://www.loom.com/share/bd9d06ef81b74f74b46ca2e80af04383?sid=3e3369ab-3e94-4d7e-8fb2-7d960cffe032 🔒

Testing scenarios

• [ ] Set the URL
• [ ] run yarn build && yarn predeploy
• [ ] check the correct value is set in PG

Final checklist

• [ ] I checked the code review guidelines
• [ ] I have added Metrics Representative as reviewer(s) if my PR invovles metrics/data/analytics related changes
• [ ] I have performed a self-review of my code, the same way I'd do it for any other team member
• [ ] I have tested all cases I listed in the testing scenarios and I haven't found any issues or regressions
• [ ] Whenever I took a non-obvious choice I added a comment explaining why I did it this way
• [ ] I added the label https://github.com/ParabolInc/parabol/labels/Skip%20Maintainer%20Review if the PR introduces only minor changes, does not contain any architectural changes or does not introduce any new patterns and I think one review is sufficient'
• [ ] PR title is human readable and could be used in changelog