[infra] Make fetch-ninja idempotent

Change-Id: I4ac33beeb7e3da29433950cf478d86d958c357e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/608017
Reviewed-by: Eric Boren <borenet@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed