paperless-ngx: re-add xdist

Without it the tests run in ~12 1/2 minutes. With it and max 8 cores
they only take 2 1/2 minutes but sometimes fail. Lets max at 4 which
takes 3 1/2 minutes. Upstream maxes at 16 by default which doesn't work for us.

pkgs/by-name/pa/paperless-ngx/package.nix

@@ -161,9 +161,12 @@ python.pkgs.buildPythonApplication rec {
   ];
 
   postPatch = ''
-    # pytest-xdist makes the tests flaky
+    # pytest-xdist with to many threads makes the tests flaky
+    if (( $NIX_BUILD_CORES > 4)); then
+      NIX_BUILD_CORES=4
+    fi
     substituteInPlace src/setup.cfg \
-      --replace-fail "--numprocesses auto --maxprocesses=16" ""
+      --replace-fail "--numprocesses auto --maxprocesses=16" "--numprocesses $NIX_BUILD_CORES"
   '';
 
   nativeBuildInputs = [
@@ -277,9 +280,13 @@ python.pkgs.buildPythonApplication rec {
     pytest-httpx
     pytest-mock
     pytest-rerunfailures
+    pytest-xdist
     pytestCheckHook
   ];
 
+  # manually managed in postPatch
+  dontUsePytestXdist = false;
+
   pytestFlagsArray = [
     "src"
   ];