python3Packages.pytest-xdist: fix hook being applied multiple times

pkgs/development/python-modules/pytest-xdist/setup-hook.sh

···
 pytestXdistHook() {
-    pytestFlagsArray+=("--numprocesses=$NIX_BUILD_CORES" "--forked" )
+    pytestFlagsArray+=(
+        "--numprocesses=$NIX_BUILD_CORES"
+        "--forked"
+    )
 }
 
 if [ -z "${dontUsePytestXdist-}" ] && [ -z "${dontUsePytestCheck-}" ]; then
-    addEnvHooks "$hostOffset" pytestXdistHook
+    preDistPhases+=" pytestXdistHook"
 fi