A request that waits behind another build, e.g. one a controller
restart orphaned, could start a full build after most of the
controller's 61-minute reply deadline was gone. The builder now waits
at most REPLY_DEADLINE - BUILD_DEADLINE - 30 s for the slot and
answers Failed, without building, past that. The 30 s covers the
forge token read, now bounded to 20 s, and the reply's trip back.
Fixes#5437