In iOS recipe, uninstall before install.

Bug: skia:7408
Change-Id: Iea55fe9199db7fc841ed0ba4a2451a84cd57213f
Reviewed-on: https://skia-review.googlesource.com/90343
Reviewed-by: Stephan Altmueller <stephana@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
4 files changed