Manually take XKB_DEFAULT_{RULES,MODEL,LAYOUT,VARIANT,OPTIONS} into account

Summary:
As kwin_wayland can have the CAP_SYS_NICE capability, libxkbcommon does not
read environment variables (see secure_getenv).
So process them here, in the same way xkb_context_sanitize_rule_names would.

BUG: 388249

Test Plan: kwin_wayland has the capability set, keyboard layout is applied correctly.

Reviewers: Plasma, graesslin

Subscribers: kwin, plasma-devel, KWin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9873