OSX builds occasionally hang

[aidanhs]

(previous tracking issues: #40061, #40571)

Over the past two weeks we've seen a hang of osx twice:

• On demandify reachability #40873 (comment)
• Rollup of 3 pull requests #41363 (comment)

The first one recovered after 45 min, the second was cancelled by us (note that it's marked as running for 4 hours - usually our builds get killed at 3!). I've let travis know about both of these.

[aidanhs]

We haven't seen a hang for a while, closing (note that this issue is different to builds not being scheduled, handled in #40672).