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.

nixos/modules/system/boot/systemd.nix

@@ -201,8 +201,13 @@ let
     ];
 
   makeJobScript = name: text:
-    let mkScriptName =  s: "unit-script-" + (replaceChars [ "\\" "@" ] [ "-" "_" ] (shellEscape s) );
-    in  pkgs.writeTextFile { name = mkScriptName name; executable = true; inherit text; };
+    let
+      scriptName = replaceChars [ "\\" "@" ] [ "-" "_" ] (shellEscape name);
+      out = pkgs.writeShellScriptBin scriptName ''
+        set -e
+        ${text}
+      '';
+    in "${out}/bin/${scriptName}";
 
   unitConfig = { config, options, ... }: {
     config = {