[pave] Move ctx.Cancel() handler inside inner xfer loop.

This should prevent the situation where the target constantly reports as
busy and we never handle a SIGTERM so it is perpetually in a broken
state.

Example: https://chrome-swarming.appspot.com/bot?id=build357-a7--bless-candy-wool-poem

Change-Id: I7ee482d2400f97c96ca47287fec2ef1380d77070
1 file changed