R: fix regression from commit cbade453 #58989

Normally the directory is created in fixup phase.

pkgs/applications/science/math/R/default.nix

@@ -72,7 +72,7 @@ stdenv.mkDerivation rec {
   # The store path to "which" is baked into src/library/base/R/unix/system.unix.R,
   # but Nix cannot detect it as a run-time dependency because the installed file
   # is compiled and compressed, which hides the store path.
-  postInstall = "echo ${which} > $out/nix-support/undetected-runtime-dependencies";
+  postFixup = "echo ${which} > $out/nix-support/undetected-runtime-dependencies";
 
   doCheck = true;
   preCheck = "export TZ=CET; bin/Rscript -e 'sessionInfo()'";