Bug 1797406 - Ignore DONTBUILD on toolchains project branch. r=taskgraph-reviewers,jcristau

The workflow for the toolchains project branch is that it's going to be
synchronized with mozilla-central, and each push is expected to trigger
new builds of the current (at the time of the push) clang upstream
trunk. If the tip of mozilla-central happens to have DONTBUILD, we still
want everything to start up.

Differential Revision: https://phabricator.services.mozilla.com/D160314