install/{consolefont,keymap,sd-vconsole}: fix XKB variable names
Use the XKB variable names that made it into the systemd 253 release. https://github.com/systemd/systemd/blob/v253/src/locale/localed-util.c#L308-L311
The XKB_* ones were replaced with XKB*.
Fixes 078562d4
Edited by nl6720