all: heartbeat buildlets always, now that reverse buildlet protocol allows it

Also, don't run shootout on linux-arm (golang/go#12623)

Change-Id: I2c85b8a04917cb0992086012eaa59b00a6cb31a4
Reviewed-on: https://go-review.googlesource.com/14582
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed