patchELF: Use correct directory

Borrowed from https://github.com/NixOS/nixpkgs/pull/13395.

pkgs/development/tools/misc/patchelf/setup-hook.sh

@@ -5,7 +5,8 @@
 fixupOutputHooks+=('if [ -z "$dontPatchELF" ]; then patchELF "$prefix"; fi')
 
 patchELF() {
-    header "shrinking RPATHs of ELF executables and libraries in $prefix"
+    local dir="$1"
+    header "shrinking RPATHs of ELF executables and libraries in $dir"
 
     local i
     while IFS= read -r -d $'\0' i; do
@@ -13,7 +14,7 @@ patchELF() {
         if ! isELF "$i"; then continue; fi
         echo "shrinking $i"
         patchelf --shrink-rpath "$i" || true
-    done < <(find "$prefix" -type f -print0)
+    done < <(find "$dir" -type f -print0)
 
     stopNest
 }