In contrast to `.service`-units, it's not possible to declare an
`overrides.conf`, however this is done by `generateUnits` for `.nspawn`
units as well. This change breaks the build if you have two derivations
configuring one nspawn unit.
This will happen in a case like this:
``` nix
{ pkgs, ... }: {
systemd.packages = [
(pkgs.writeTextDir "etc/systemd/nspawn/container0.nspawn" ''
[Files]
Bind=/tmp
'')
];
systemd.nspawn.container0 = {
/* ... */
};
}
```
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