setup-hooks: do not pass missing dirs to find (close #15405)

find fails when called with an inexistent search path.
That situation may arise when the output is created after by a postFixup hook.
vcunat amended the PR by clarifying one more `return` to `return 0`.

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

@@ -6,6 +6,8 @@ fixupOutputHooks+=('if [ -z "$dontPatchELF" ]; then patchELF "$prefix"; fi')
 
 patchELF() {
     local dir="$1"
+    [ -e "$dir" ] || return 0
+
     header "shrinking RPATHs of ELF executables and libraries in $dir"
 
     local i