This patch adds per task rebuild counts to mach try perf. Previously a category like "critical desktop" could not be mixed with other categories (the default of 20 rebuilds on sp3 would be dropped to 1, for example) Now, categories can specify per-task-rebuild inside try_task_config. Critical desktop category now gives sp3 20 retriggers by default regardless of what other categories are selected alongside it. Differential Revision: https://phabricator.services.mozilla.com/D296240