Avoid modifying the original klipper.service

[dflemstr]

We can use systemd's drop-in support to add a ExecStartPre step without modifying the original file. This removes the risk of the sed approach possibly messing up the syntax of the file (although unlikely), and additionally preserves the change even if a new version of klipper.service is shipped from upstream.

[Drachenkaetzchen]

I haven't had time to check+review so far, but I haven't forgotten!