fix: add ability to specify your own TF_TOKEN

See merge request https://gitlab.com/gitlab-org/terraform-images/-/merge_requests/179



Merged-by: Tiger Watson <twatson@gitlab.com>
Approved-by: Tiger Watson <twatson@gitlab.com>
Approved-by: Timo Furrer <tfurrer@gitlab.com>
Co-authored-by: Timo Furrer <tfurrer@gitlab.com>
Co-authored-by: Tobias Mueller <tm@eriksterck.de>
Co-authored-by: Tobias <tobias.mueller83@gmail.com>