Baseline: 2a88c8da7783490ef6192336b091910cd662cc67

Cherry picks:

   + 70691f2b76be0b9530e49c3df18356925843b465:
     Make the Linux sandbox work with ActionInputs with absolute
     "exec paths".
   + 40b340722a97bd3ce1692047f173547203bb7452:
     Fix NPE in signal ignoring logic.

This release contains contributions from many people at Google, as well as aryeh, Fabian Meumertzheim, Gaspare Vitta, John Hinnegan, Roger Hu, Yi Cheng.