- Don't fail on `kill $cachixPid` when cachix has already exited.
This fixes some failing CI builds.
- Stop the cachix background worker before the final `cachix push`.
This can avoid unneeded reuploads.
Use the coreutils version of tail on cirrus which supports
argument --pid.
Needed by the following commits.
Also, don't use the cachix cache as a substituter for local, non-CI
builds. This obviates the need for the 'untrusted' warning in build.sh.