python3Packages.sphinxHook: Avoid propagating sphinx

Fixes some side effects of #249157 (see #255810)

pkgs/development/interpreters/python/hooks/default.nix

···
   sphinxHook = callPackage ({ makePythonHook, installShellFiles }:
     makePythonHook {
       name = "python${python.pythonVersion}-sphinx-hook";
-      propagatedBuildInputs = [ pythonForBuild.pkgs.sphinx installShellFiles ];
+      propagatedBuildInputs = [ installShellFiles ];
+      substitutions = {
+        sphinxBuild = "${pythonForBuild.pkgs.sphinx}/bin/sphinx-build";
+      };
     } ./sphinx-hook.sh) {};
 }

pkgs/development/interpreters/python/hooks/sphinx-hook.sh

···
 
     for __builder in "${__sphinxBuilders[@]}"; do
         echo "Executing sphinx-build with ${__builder} builder"
-        sphinx-build -M "${__builder}" "${__sphinxRoot}" ".sphinx/${__builder}" -v
+        @sphinxBuild@ -M "${__builder}" "${__sphinxRoot}" ".sphinx/${__builder}" -v
     done
 
     runHook postBuildSphinx