buildlet: make exec fail with a remote error if no headers after 5 seconds

The reverse buildlets' RoundTrip are hanging, which is its own problem,
but this calling code should be robust and time out anyway.

Change-Id: Id9e3e1d9feb6ffa58cc0995d0623bd90845bb9d6
Reviewed-on: https://go-review.googlesource.com/10847
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed