Build .changes task scheduled on workers with no architecture support

Hi,

I created a build_pipeline pipeline for a .dsc artifact on stretch. That requires a .changes, so a Make .changes file task was automatically created, but it was scheduled on an arm64 worker. Since stretch doesn't support arm64, it failed with:

Failed to compute dynamic data: "'freexian@debian:environments' has no item 'match:codename=stretch-security:architecture=arm64:format=tarball:backend=unshare:variant='"

The task should be scheduled on a worker with an available environment.