nixos/image: write systemd-repart output to $out/repart-output.json

Write the output of `systemd-repart` as a JSON file to
`$out/repart-output.json`.

Depending on the repart configuration, the output of `systemd-repart`
contains important information, for example, when creating verity
partitions:

> The verity root hash itself will be included in the output of
> systemd-repart.

See `Verity=` in repart.d(5).

nixos/modules/image/repart.nix

@@ -195,7 +195,9 @@ in
           --seed="${cfg.seed}" \
           --definitions="$amendedRepartDefinitions" \
           --split="${lib.boolToString cfg.split}" \
-          image.raw
+          --json=pretty \
+          image.raw \
+          | tee repart-output.json
       '';
 
     meta = {