Explore alternative solutions for very large environment variable passing for subprojects

A customer recently reported a weird error when running k6 where the error came from the OS and has been linked with environment variables that are too large.

At the time this was confusing but after review we do have a potentially very large environment variable in ENVIRONMENT_MANY_GROUPS_AND_PROJECTS. Currently we list all subprojects in this env var and pass to k6.

We probably shouldn't rely on such massive environment variables to avoid the reported issue. Task is to explore alternative solutions such as moving the selection logic down to k6 to do in memory.