cmd/buildlet/stage0: stop using deprecated --reverse flag to buildlet
cmd/buildlet/stage0: stop using deprecated --reverse flag to buildlet

Updates golang/go#21260

Change-Id: Ic4d96ef3984017944c5a9af087ea7010e5193f32
Reviewed-on: https://go-review.googlesource.com/52650
Reviewed-by: Jessica Frazelle <b18dc8896bab423463090ef48bfaba8e360ec777@google.com>

golang/build,golang/build,golang/build,golang/build