Due to OS version detection, ubuntu and debian are detected as linux, limiting maximum the version of mongo to 4.0

**Describe the bug**
The OS detection will detect ubuntu and debian as 'linux'
As such the URLS for download will be limited to a maximum of version 4.0

**To Reproduce**
Steps to reproduce the behavior:
1. Use ubuntu server/desktop
2. Try to set the version using env variables to 6.0
3. Run with pymongo_inmemory
4. Check Mongo server version. It will be 4.0

**Expected behavior**
Version 6.0 should be installed

**Logs**
Not applicable

**Screenshots**
Not applicable

**Context:**
- OS: Ubuntu 22.04
- Version of pymongo_inmemory: 0.4.2

Reproduce the issue on Ubuntu 22.04 using the environment variables for MongoDB 6.0, then inspect the OS detection and MongoDB download URL selection. Done means Ubuntu and Debian select a compatible download so pymongo_inmemory runs MongoDB 6.0 instead of being limited to 4.0.

Written by the indexing model from the issue text.