maint(common): improve handling of FirstVoices builds

From https://github.com/keymanapp/keyman/pull/14353/files#r2220239240:

> So instead of that, perhaps we can create a new env var, and just maintain that as well as the two RELEASE_OEM vars until 19.0 goes to stable? I would maybe call it BUILD_OEM_PROJECTS=fv (and we add comma separated projects OEM later if we wanted to), and then add a parameter to the TC scripts, e.g. --oem-projects=%env.BUILD_OEM_PROJECTS% (which we could translate to --fv internally if we wanted to).

- modify TC build scripts to accept `--oem-projects` parameter instead of `--fv`
- for now, set `RELEASE_OEM` and `RELEASE_OEM_FIRSTVOICES` env variables in build script (eventually we might want to get rid of these variables)
- add `BUILD_OEM_PROJECTS` env variable to TC build config
- modify TC build config to pass `--oem-projects` instead of `--fv`