Taskcluster infrastructure/ job should always run all three browsers

[gsnedders]

Currently, the infrastructure job ends upon the first failure — e.g., https://community-tc.services.mozilla.com/tasks/C5zpv5rfSSiRwFlC5ziQaA ran only Chrome, and never ran Firefox or Firefox-on-Android.

We should always run all three, even if one fails.

Otherwise, pre-existing failures on master affecting one will cause us to lose coverage of all three.