x/build/cmd/coordinator: add timeout to make.bash

[bradfitz]

It seems the build coordinator doesn't have a timeout on the make.bash phase?

This Mac builder is stuck in make.bash for over 4 hours:

darwin-amd64-10_14 rev 2566e21f; running; http://macstadium_host07b reverse peer macstadium_host07b/207.254.3.58:60202 for host type host-darwin-10_14, 4h48m28s ago
  2019-11-04T23:20:00Z checking_for_snapshot 
  2019-11-04T23:20:01Z finish_checking_for_snapshot after 377.4ms
  2019-11-04T23:20:01Z get_buildlet 
  2019-11-04T23:20:01Z wait_static_builder host-darwin-10_14
  2019-11-04T23:20:01Z finish_wait_static_builder after 0s; host-darwin-10_14
  2019-11-04T23:20:01Z clean_buildlet http://macstadium_host07b reverse peer macstadium_host07b/207.254.3.58:60202 for host type host-darwin-10_14
  2019-11-04T23:20:01Z finish_clean_buildlet after 32.1ms; http://macstadium_host07b reverse peer macstadium_host07b/207.254.3.58:60202 for host type host-darwin-10_14
  2019-11-04T23:20:01Z finish_get_buildlet after 276.1ms
  2019-11-04T23:20:01Z using_buildlet macstadium_host07b
  2019-11-04T23:20:01Z write_version_tar 
  2019-11-04T23:20:01Z get_source 
  2019-11-04T23:20:05Z finish_get_source after 3.9s
  2019-11-04T23:20:05Z write_go_src_tar 
  2019-11-04T23:20:11Z finish_write_go_src_tar after 6.16s
  2019-11-04T23:20:11Z make_and_test 
  2019-11-04T23:20:11Z make src/make.bash
 +17297.4s (now)