Building the MSIs can take quite a while. Add some logging to keep the
watchdog happy.

For golang/go#54134.

Change-Id: I6178323f6e34a6ecd409370e9e396f683a6cdb9f
Reviewed-on: https://go-review.googlesource.com/c/build/+/420542
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

In preparation for adding a watchdog timer, use a common helper function
that can reset the watchdog automatically.

For golang/go#54134.

Change-Id: I257a97cfeb540e90b0c005e255c40ee1d054bae0
Reviewed-on: https://go-review.googlesource.com/c/build/+/420540
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>

Rather than buffering buildlet output and only showing it in case of
error, stream it to a user-supplied Writer. In relui, buffer 10
seconds of output, or up to 1MB, then dump everything up to the most
recent newline to the logger.

For golang/go#54134.

Change-Id: Ie0eb9af3aa6c3455afd03b358722924e38058063
Reviewed-on: https://go-review.googlesource.com/c/build/+/420541
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Buildlet tasks weren't resetting the watchdog timer because they
bypassed the TaskContext's Printf method. Ideally the Logger would be
unexported to prevent this mistake, but we construct TaskContext's in
tests all over the place. For now, I got rid of all the existing reads
and we can worry about it later if it comes up again.

For golang/go#54134.

Change-Id: I43c38cabd9bff298c72affaa1792b0ac0ad2cb5c
Reviewed-on: https://go-review.googlesource.com/c/build/+/422597
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>