nixos/repart-image: don't pass seed when it's null, update documentation

From systemd-repart(8):

If not specified it is attempted to read the machine ID from the host
(or more precisely, the root directory configured via --root=) and use
it as seed instead, falling back to a randomized seed otherwise. Use
--seed=random to force a randomized seed.

The option documentation now suggests the special value "random", for
a random seed rather than relying on the implicit fact that there is
no machine ID available in the build sandbox, causing repart to fallback
on generating a random seed.

Fixes #419331

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