[libkworkspace] Only update env vars with alphanumeric_ names

It gives issues with systemd to try and pass it env var names with eg.
'%' in them. That to such a degree that if invalid names are passed,
none are set [1]. This change ensures compatibility by skipping any
non-alphanumerical (and _) variable names.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1754395