[pkgctl] allow suppying the gitlab token via env var

This would allow to supply the gitlab tokens via the env var DEVTOOLS_GITLAB_TOKEN and therefore allow users to choose whatever program they want to fill this env var.

The idea is inspired by gh and arch-signoff for which I both have the following two aliases in my config to use the password manager of my choice:

alias signoff='ARCHWEB_USERNAME="gromit" ARCHWEB_PASSWORD=$(gopass show --password websites/www.archlinux.org/gromit) signoff'
alias gh='GH_TOKEN=$(gopass show --password "misc/github-token") gh'

closes #113 (closed) related to &7 (closed)