lib/tests/release.nix: Fix build if `impure.nix` returns a cross pkgs

In https://www.github.com/NixOS/nixpkgs/pull/341067 `ofborg-eval-lib-tests`
fails because it uses `pkgsHostTarget` of the cross pkgs

```diff
diff --git a/pkgs/top-level/impure.nix b/pkgs/top-level/impure.nix
index 4d847e280f4b..7f0099b254f0 100644
--- a/pkgs/top-level/impure.nix
+++ b/pkgs/top-level/impure.nix
@@ -86,4 +86,6 @@ assert args ? system -> !(args ? localSystem);

import ./. (builtins.removeAttrs args [ "system" ] // {
inherit config overlays localSystem;
-})
+} // (if ((localSystem.system or localSystem) == "x86_64-linux" || (localSystem.system or localSystem) == "aarch64-linux") then {
+ crossSystem = { config = "aarch64-unknown-linux-gnu"; };
+ } else {}) )
```

Changed files
+8 -3
lib
tests
+8 -3
lib/tests/release.nix
···
{ # The pkgs used for dependencies for the testing itself
# Don't test properties of pkgs.lib, but rather the lib in the parent directory
pkgs ? import ../.. {} // { lib = throw "pkgs.lib accessed, but the lib tests should use nixpkgs' lib path directly!"; },
+
# For testing someone may edit impure.nix to return cross pkgs, use `pkgsBuildBuild` directly so everything here works.
+
pkgsBB ? pkgs.pkgsBuildBuild,
nix ? pkgs-nixVersions.stable,
nixVersions ? [ pkgs-nixVersions.minimum nix pkgs-nixVersions.latest ],
-
pkgs-nixVersions ? import ./nix-for-tests.nix { inherit pkgs; },
+
pkgs-nixVersions ? import ./nix-for-tests.nix { pkgs = pkgsBB; },
}:
let
lib = import ../.;
testWithNix = nix:
-
import ./test-with-nix.nix { inherit lib nix pkgs; };
+
import ./test-with-nix.nix {
+
inherit lib nix;
+
pkgs = pkgsBB;
+
};
in
-
pkgs.symlinkJoin {
+
pkgsBB.symlinkJoin {
name = "nixpkgs-lib-tests";
paths = map testWithNix nixVersions;
}