Bug 1960906: key treescript workers by project first, then release-level r=releng-reviewers,jcristau
Over in https://bugzilla.mozilla.org/show_bug.cgi?id=1752111 a block was [added to the treescript worker definition](https://searchfox.org/mozilla-central/rev/08b2a1a770688df19a5a43fd89fb63b34bb2d6a6/taskcluster/config.yml#781,783-787) that uses level 3 workers when the `release-level` is `staging`, but only for `autoland`. This was done because [`autoland` is not a RELEASE_PROJECT](https://searchfox.org/mozilla-central/rev/08b2a1a770688df19a5a43fd89fb63b34bb2d6a6/taskcluster/gecko_taskgraph/util/attributes.py#14), but we need to use level 3 workers in order to be able to push to it.
In an ideal world, we'd add `autoland` to `RELEASE_PROJECTS`. Unfortunately, doing so has other implications, such as causing the `upload-symbols` tasks to run whenever shippable builds are run there, which we don't want.
(treescript is going away soon, but we'll need a similar block for landoscript in https://bugzilla.mozilla.org/show_bug.cgi?id=1948315. I'd like to land this for treescript first to shake out an subtle issues that may arise ahead of time.)
This change is really just a readability change to avoid the strange look of having a level 3 worker inside of a `staging` key block.
Differential Revision: https://phabricator.services.mozilla.com/D245734
89d429fee4