systemd: Simplify unit script names

Current journal output from services started by `script` rather than
`ExexStart` is unreadable because the name of the file (which journalctl
records and outputs) quite literally takes 1/3 of the screen (on smaller
screens).

Make it shorter. In particular:

* Drop the `unit-script` prefix as it is not very useful.
* Use `writeShellScriptBin` to write them because:
  * It has a `checkPhase` which is better than no checkPhase.
  * The script itself ends up having a short name.