Re-using Python virtual environments on a build server
It's generally a best practice to make your builds idempotent.  Leaving artifacts behind only opens up opportunities for dependency management issues- and resolving those in a resilient way is exactly why you're using a build server.  I suggest you take a look at WHY your Python builds are running so slow- if you discover this is because of an overabundance of packages in each project, this might be optimizable.  If you discover this is because of server performance issues, those are easily fixable with more resources.  If you find you are blocked by network throughput, consider using an artifact repository to cache packages locally.  But building persistent virtualenvs seems like a recipe for problems that could be unpredictable when moving to another server, and extremely frustrating to troubleshoot.