ci: move baseline version detection into separate job

ci: move baseline version detection into separate job

Multiple CI jobs may utilize a baseline version, i.e. the version that
the current code should be tested against when doing comparative
testing. To avoid repeating the non-trivial detection of the baseline
version, move it into a separate job which creates an environment file
that subsequent jobs may require via `needs` option. It is then possible
to use the variable(s) defined in the script section of the new job.