ci: Make scan script more versatile

This makes the bin/scan_gitlab.sh script more versatile. The new help output explains:

Usage: scan_gitlab.sh <REF> [SEMGREP_OPTIONS...]

Scans the GitLab repository at commit REF. Any additional arguments are passed to semgrep.

In CI, the GitLab repository is cloned at REF.

To run this locally, first symlink the GitLab repository to "gitlab". For example:

    ln -s <path_to_gitlab_repo> gitlab

NOTE: This script will mutate the locally linked GitLab repository in the following ways:
      - Updates the origin/master remote branch
      - Checks out REF (and attempts to check out the original ref before exiting)

Part of #16 (closed).