setup-hooks/make-symlinks-relative.sh: match what other hooks do so

the hook doesn't output the following if $prefix doesn't exist

find: '/nix/store/...': No such file or directory

some other hooks that do this: prune-libtool-files.sh, audit-tmpdir.sh separate-debug-info.sh

pkgs/build-support/setup-hooks/make-symlinks-relative.sh

@@ -6,8 +6,8 @@ postFixupHooks+=(_makeSymlinksRelative)
 _makeSymlinksRelative() {
     local symlinkTarget
 
-    if [ -n "${dontRewriteSymlinks-}" ]; then
-        return 0
+    if [ "${dontRewriteSymlinks-}" ] || [ ! -e "$prefix" ]; then
+       return
     fi
 
     while IFS= read -r -d $'\0' f; do