audit-tmpdir.sh: fix on darwin

compress-man-pages.sh uses `find ... | while ...; do...` so this works
on darwin

vcunat says the following in https://github.com/NixOS/nixpkgs/commit/94d164dd7#r22030369

> Unfortunately, process substitution doesn't work during early darwin bootstrap. http://hydra.nixos.org/build/52433342 We've run into that before.
> I think it should work to have `find ... | while ...; do...`

pkgs/build-support/setup-hooks/audit-tmpdir.sh

@@ -16,7 +16,7 @@ auditTmpdir() {
     header "checking for references to $TMPDIR/ in $dir..."
 
     local i
-    while IFS= read -r -d $'\0' i; do
+    find "$dir" -type f -print0 | while IFS= read -r -d $'\0' i; do
         if [[ "$i" =~ .build-id ]]; then continue; fi
 
         if isELF "$i"; then
@@ -35,7 +35,7 @@ auditTmpdir() {
             fi
         fi
 
-    done < <(find "$dir" -type f -print0)
+    done
 
     stopNest
 }