novnc: correctly install and point to `websockify`

`utils/novnc_proxy` tries to download and run the `websockify` package
in the directory where the script is located, which doesn't work because
the nix store is read-only. This patches the script to point to
nix-installed `websockify`.

pkgs/applications/networking/novnc/default.nix

···
-{ lib, stdenv, fetchFromGitHub }:
+{ lib, python3, stdenv, substituteAll, fetchFromGitHub }:
 
 stdenv.mkDerivation rec {
   pname = "novnc";
···
     sha256 = "sha256-Z+bks7kcwj+Z3uf/t0u25DnGOM60QhSH6uuoIi59jqU=";
   };
 
-  patches = [ ./fix-paths.patch ];
+  patches = with python3.pkgs; [
+    (substituteAll {
+      src = ./websockify.patch;
+      inherit websockify;
+    })
+  ] ++ [ ./fix-paths.patch ];
 
   postPatch = ''
     substituteAllInPlace utils/novnc_proxy

pkgs/applications/networking/novnc/websockify.patch

···
+diff --git a/utils/novnc_proxy b/utils/novnc_proxy
+index 0365c1e..7eba2db 100755
+--- a/utils/novnc_proxy
++++ b/utils/novnc_proxy
+@@ -167,7 +167,7 @@ if [[ -d ${HERE}/websockify ]]; then
+ 
+     echo "Using local websockify at $WEBSOCKIFY"
+ else
+-    WEBSOCKIFY_FROMSYSTEM=$(which websockify 2>/dev/null)
++    WEBSOCKIFY_FROMSYSTEM="@websockify@/bin/websockify"
+     WEBSOCKIFY_FROMSNAP=${HERE}/../usr/bin/python2-websockify
+     [ -f $WEBSOCKIFY_FROMSYSTEM ] && WEBSOCKIFY=$WEBSOCKIFY_FROMSYSTEM
+     [ -f $WEBSOCKIFY_FROMSNAP ] && WEBSOCKIFY=$WEBSOCKIFY_FROMSNAP