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.
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