Compute agent build version from release tags

[robbkidd]

Attempted in #186 but reverted in #206 because the GitHub action/checkout step does not pull tags into the build environment by default (like what occurs in our Circle CI pipelines).

Implementing this would make the release process for this project match our other projects more closely where we do not have to hard-code the version in source and development builds have non-release version numbers baked in that make their provenance more clear.