Rename `TF_GITLAB_SOURCED` variable to `GITLAB_TOFU_SOURCED`.

This change set renames the `TF_GITLAB_SOURCED` environment variable to
`GITLAB_TOFU_SOURCED` and document the existence of the variable
accordingly in the README.
The reason for renaming it is that the `TF_` variable prefix is "owned"
by OpenTofu / Terraform and not GitLab Tofu. Therefore, we make an
effort to rename these variables to have the `GITLAB_TOFU_` prefix
instead.

Changelog: changed