Build job has failed to start on AppVeyor

[pietroalbini]

We had a couple builds in the past few days where AppVeyor failed to start a build, giving us an empty log and a "Build job has failed to start, but backup cloud is not configured." message.

• Rollup of 6 pull requests #61852 (comment) (job - 2019-06-14T23:08:24Z)
• Update cargo #61771 (comment) (log - 2019-06-17T23:30:33Z)

[pietroalbini]

Opened an AppVeyor support ticket.

[pietroalbini]

Seems like the issue only appeared on their GCP builders, so I opened a PR (#61972) to switch to their Hyper-V ones, as they recommended.

[pietroalbini]

AppVeyor said the issue might start to happen again.

[pietroalbini]

Well, this is not a problem anymore.