Move uploading maven artifacts to the inner loop

Unclear why they were ourside of the inner loop to start with,
seems like some of the variables like `out_dir` were not valid.

Change-Id: Idd861646b7095893ff707655df6794cb83a8e05f
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/14500
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Kaushik Iska <kaushikiska@google.com>