linux-builder: make compatible with cross-arch builder package

Before this commit, aarch64 users building the following configuration
would end up with an aarch64-linux builder, while after it, they get the
x86_64-linux builder they expect:
```nix
 nix.linux-builder = {
  enable = true;
  package = pkgs.darwin.linux-builder-x86_64;
};
```

Before, in order to get an x86_64-linux builder, they would have needed
to use this configuration instead:
```nix
 nix.linux-builder = {
  enable = true;
  config.nixpkgs.hostPlatform = "x86_64-linux";
  systems = ["x86_64-linux"];
};
```

The reason for this is that the linux-builder module calls `override` on
the package option, and the `linux-builder-x86_64` package is also
defined using override:
```nix
linux-builder-x86_64 = linux-builder.override {
  modules = [ { nixpkgs.hostPlatform = "x86_64-linux"; } ];
};
```

The module was effectively discarding the `nixpkgs.hostPlatform` option.

Example issue: https://github.com/NixOS/nixpkgs/issues/313784

1 files changed, 9 insertions, 5 deletions

diff --git a/modules/nix/linux-builder.nix b/modules/nix/linux-builder.nix
index 176d69e..b0c3cd6 100644
--- a/modules/nix/linux-builder.nix
+++ b/modules/nix/linux-builder.nix
@@ -7,9 +7,11 @@ let
 
   cfg = config.nix.linux-builder;
 
-  builderWithOverrides = cfg.package.override {
-    modules = [ cfg.config ];
-  };
+  builderWithOverrides = cfg.package.override (previousArguments: {
+    # the linux-builder packages require a list `modules` argument, so it's
+    # always non-null.
+    modules = previousArguments.modules ++ [ cfg.config ];
+  });
 
   # create-builder uses TMPDIR to share files with the builder, notably certs.
   # macOS will clean up files in /tmp automatically that haven't been accessed in 3+ days.
@@ -133,8 +135,10 @@ in
 
     systems = mkOption {
       type = types.listOf types.str;
-      default = [ "${stdenv.hostPlatform.uname.processor}-linux" ];
-      defaultText = literalExpression ''[ "''${stdenv.hostPlatform.uname.processor}-linux" ]'';
+      default = [ builderWithOverrides.nixosConfig.nixpkgs.hostPlatform.system ];
+      defaultText = ''
+        The `nixpkgs.hostPlatform.system` of the build machine's final NixOS configuration.
+      '';
       example = literalExpression ''
         [
           "x86_64-linux"