hydra: only set buildMachinesFiles when nix.buildMachines is defined

This applies [hydra PR #432](https://github.com/NixOS/hydra/pull/432)
to the NixOS module in nixpkgs:

```
commit 4efd078977e5ea20e1104783efc324cba11690bc
Author: Bas van Dijk <v.dijk.bas@gmail.com>
Date:   Sun Dec 11 15:35:38 2016 +0100

    Only set buildMachinesFiles when nix.buildMachines is defined
```
This commit is contained in:
Bas van Dijk 2020-01-14 12:32:40 +01:00
parent e306ee4a56
commit ad554b3e4e

View File

@ -167,7 +167,7 @@ in
buildMachinesFiles = mkOption {
type = types.listOf types.path;
default = [ "/etc/nix/machines" ];
default = optional (config.nix.buildMachines != []) "/etc/nix/machines";
example = [ "/etc/nix/machines" "/var/lib/hydra/provisioner/machines" ];
description = "List of files containing build machines.";
};