app-admin/toolbox: fix the upstream default reference name

The upstream moved from "master" to "main" default branch which
needs to be reflected in the ebuild file.

BUG=b/189493262
TEST=presubmit
RELEASE_NOTE=None

Change-Id: I80f7fd4b63c5e21f2d80551cdf50de87161c2d53
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/17230
Reviewed-by: Roy Yang <royyang@google.com>
Reviewed-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
2 files changed