startkde: Avoid pushing shell variables to UpdateLaunchEnvJob

Currently, `startplasma` filters out shell variables when loading env
scripts and systemd environment variables. However, this is insufficient
if `startplasma` itself was run from within a shell, which is the case
with X11 sessions in some distros. This causes `SHELL=/bin/bash` to be set
in applications' launch environments, which may cause unexpected behavior.
One of the most visible examples is Konsole, where the default profile
will load bash (due to `$SHELL`) instead of the user's actual shell.

This commit adds an additional check to filter out shell variables
before they are sent to `UpdateLaunchEnvJob`.