Merge pull request #124946 from NixOS/backport-123253-to-release-21.05

[Backport release-21.05] vapoursynth: improve plugin dependency resolution

pkgs/development/libraries/vapoursynth/plugin-interface.nix

@@ -8,10 +8,11 @@ plugins: let
 
   getRecursivePropagatedBuildInputs = pkgs: lib.flatten
     (map
-      (pkg: pkg.propagatedBuildInputs ++ (getRecursivePropagatedBuildInputs pkg.propagatedBuildInputs))
+      (pkg: let cleanPropagatedBuildInputs = lib.filter lib.isDerivation pkg.propagatedBuildInputs;
+        in cleanPropagatedBuildInputs ++ (getRecursivePropagatedBuildInputs cleanPropagatedBuildInputs))
       pkgs);
 
-  deepPlugins = plugins ++ (getRecursivePropagatedBuildInputs plugins);
+  deepPlugins = lib.unique (plugins ++ (getRecursivePropagatedBuildInputs plugins));
 
   pluginsEnv = buildEnv {
     name = "vapoursynth-plugins-env";