tinydm-run-session: source profile

Restore functionallity of sourcing profile, as it was removed in autologin: https://git.sr.ht/~kennylevinsen/autologin/commit/3f3a40c773dbcd229de66a006a2f2a17c6608afa

In postmarketOS, we have a previous version of autologin packaged currently. The plan is, after this MR is merged, tag a new version, then upstream autologin 1.0.0 and tinydm with this fix and fixed elogind dependency (https://gitlab.com/postmarketOS/pmaports/-/merge_requests/1775) to alpine.

EDIT: verified that this works.