feat: Allow setting git-user-email with env variables

What does this MR do and why?

This MR allows to set the git-user-email via an ENV variable (like the rest of the flags). It also adds a fallback to the old default value to not break the existing workflow. The environment variable is set by GitLab alongside the StartWorkflowRequest (see this MR).

Related issues

gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#1487 (closed)

gitlab-org/gitlab!206557 (merged)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Related to gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#1487 (closed)