Enforces gsi_* prefix when fetching gsi artifacts. am: 45ebb74b40 am: 34e266057e am: c49fb6eba9

Original change: https://android-review.googlesource.com/c/platform/tools/treble/+/1842894

Change-Id: I89d1def7a3ceed18653537b635ee689ec50d3ec0