add an out-of-source build to CI

add an out-of-source build to CI

The existing CI tasks run an uncommon work flow: building distributable packages
on each operating system. A more common work flow end users will be following
with the portable source tarball is simply building and installing it. This
change adds a job that does this, to provide a safeguard against breaking this
work flow in future.

The added job runs on Ubuntu 21.04. The intent is to bump this job to the latest
each time there is a new Ubuntu release. There is no companion test job for the
build job because we assume the existing test jobs are providing sufficient
coverage.

Closes #2073.