Use canonicalized runner debug symbol path for fuchsia.

There is always a 1:1 mapping of `$root_out_dir` and `flutter-debug-symbols-${flutter_runtime_mode}-${target_os}-${target_cpu}` as `$root_out_dir` is simply `${target_os}_${flutter_runtime_mode}_${target_cpu}`.
This means that we don't have to worry about any file/directory collisions.

http://go/fepr/26426 needs to land before `recipes-with-led` can pass.

Bug: fxbug.dev/76223
Change-Id: Idb53d4f2f5d957b5636792a79468413d0d746ed5
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/13960
Reviewed-by: Chase Latta <chaselatta@google.com>
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Darren Chan <chandarren@google.com>