GitLab driver branch fetch is using deprecated GitLab CI environment variables

## Summary / Background

The GitLab driver is using a deprecated environment variable to fetch branch name (namely `CI_BUILD_REF_NAME`).

## Scope

Users using CML on GitLab 16.x.

## Assumptions

Should be replaced by either `CI_COMMIT_REF_NAME` or `CI_COMMIT_BRANCH`.

## Open Questions

-

## Blockers / Dependencies

-

## General Approach

In GitLab CI:
`cml pr .`

Ends up with:
```
{"level":"error","message":"git,checkout,-B,,c8ea18db3c47a566b6939be8b8c2831dbc5665df\n\t\n\tfatal: '' is not a valid branch name\n","stack":"Error: git,checkout,-B,,c8ea18db3c47a566b6939be8b8c2831dbc5665df\n\t\n\tfatal: '' is not a valid branch name\n\n at /usr/lib/node_modules/@dvcorg/cml/src/utils.js:38:18\n at ChildProcess.exithandler (node:child_process:410:5)\n at ChildProcess.emit (node:events:513:28)\n at maybeClose (node:internal/child_process:1100:16)\n at Process.ChildProcess._handle.onexit (node:internal/child_process:304:5)"}
```

## Timelines

I can push a fix by the end of the week.