eval.nix: Accept path-likes in meta.nixpkgs as well

This fixes usage with Niv.

src/nix/hive/eval.nix

@@ -80,7 +80,7 @@ let
       }
     '';
   in
-    if typeOf pkgConf == "path" then
+    if typeOf pkgConf == "path" || (typeOf pkgConf == "set" && pkgConf ? outPath) then
       if hermetic then throw (uninitializedError "a path to Nixpkgs")
       # The referenced file might return an initialized Nixpkgs attribute set directly
       else mkNixpkgs configName (import pkgConf)

src/nix/hive/tests/mod.rs

@@ -319,6 +319,22 @@ fn test_nixpkgs_system() {
     "#);
 }
 
+#[test]
+fn test_nixpkgs_path_like() {
+    TempHive::valid(r#"
+      {
+        meta = {
+          nixpkgs = {
+            outPath = <nixpkgs>;
+          };
+        };
+        test = { pkgs, ... }: {
+          boot.isContainer = true;
+        };
+      }
+    "#);
+}
+
 #[test]
 fn test_nixpkgs_overlay_meta_nixpkgs() {
     // Only set overlays in meta.nixpkgs