Make combineWrappersInfo a nix function that returns a shell command

rfcs/0075-declarative-wrappers.md

@@ -333,17 +333,36 @@ so:
 
 ```nix
   # Propagate GST plugins' path
-  postInstall = ''
-    echo "${builtins.toJSON {
+  postInstall = makeWrapperAuto.combineWrappersInfo {
+    envInfo = {
       GST_PLUGIN_SYSTEM_PATH_1_0 = [
         # @out@ should be expanded by `combineWrappersInfo` to what's in `$out`, see:
         # https://github.com/NixOS/nixpkgs/pull/85103#issuecomment-613071343
         "@out@/lib/gstreamer-1.0"
       ];
-    }}" | combineWrappersInfo
+    };
+  };
   '';
 ```
 
+The above will put a shell command in `postInstall` that's generated by
+`combineWrappersInfo` that will put the environment information into
+`wrappers.json`.
+
+It should be possible to tell `combineWrappersInfo` to use a symlink directory
+instead of a big environment variable, by using for example: 
+
+Other nix attributes along `envInfo` such as:
+
+```nix
+  envInfo = {
+    # ...
+  };
+  symlink = {
+    THIS_ENV_VAR = "to/this-directory/in-out";
+  };
+```
+
 `wrapQtAppsHook` and `wrapGAppsHook` should be replaced with `makeWrapperAuto`
 which enables removing frequent workarounds such as: