discourse: Change the path to the auto generated plugin assets

Change the path to the auto generated plugin assets, which defaults to
the plugin's directory and isn't writable at the time of asset
generation.

pkgs/servers/web-apps/discourse/auto_generated_path.patch

@@ -0,0 +1,13 @@
+diff --git a/lib/plugin/instance.rb b/lib/plugin/instance.rb
+index 380a63e987..b2ce7fa982 100644
+--- a/lib/plugin/instance.rb
++++ b/lib/plugin/instance.rb
+@@ -403,7 +403,7 @@ class Plugin::Instance
+   end
+ 
+   def auto_generated_path
+-    File.dirname(path) << "/auto_generated"
++    "#{Rails.root}/public/assets/auto_generated_plugin_assets/#{name}"
+   end
+ 
+   def after_initialize(&block)

pkgs/servers/web-apps/discourse/default.nix

@@ -158,6 +158,11 @@ let
       # Use the Ruby API version in the plugin gem path, to match the
       # one constructed by bundlerEnv
       ./plugin_gem_api_version.patch
+
+      # Change the path to the auto generated plugin assets, which
+      # defaults to the plugin's directory and isn't writable at the
+      # time of asset generation
+      ./auto_generated_path.patch
     ];
 
     # We have to set up an environment that is close enough to
@@ -244,6 +249,11 @@ let
       # Use mv instead of rename, since rename doesn't work across
       # device boundaries
       ./use_mv_instead_of_rename.patch
+
+      # Change the path to the auto generated plugin assets, which
+      # defaults to the plugin's directory and isn't writable at the
+      # time of asset generation
+      ./auto_generated_path.patch
     ];
 
     postPatch = ''