sweet: add -clean flag to get

This is to help clean up old copies of the assets, which use a lot of
disk space.

Change-Id: Id2c97f270343a0c524eea7a31db8f6e48c059945
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/680496
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>