Bug 1646834: [scriptworker-canary] Split linux signing tasks to handle partials; r=rail

Since partials have started verifying signatures, the partial task has been failing in
mach try scriptworker. Since we are not concerned with the partial task itself,
split the tasks into two groups, so that it does not need to run.

Differential Revision: https://phabricator.services.mozilla.com/D83370