don't include coverage commands in windows tox runs
The coverage commands are periodically flaky on Windows: https://dev.azure.com/elementl/dagster/_build/results?buildId=10088&view=logs&j=4e8e61ab-cdcc-5fb5-0caf-ebfe77ae9f01&t=a197a35e-3187-5338-927a-3831ce363aeb&l=75
and probably shouldn't be writing to the same coverage site as unix anyway since they include different tests and codepaths?
So for now, disable coverage on windows.
Test Plan: BK + Azure, Verify that coverage is still being logged on unix BK, and that coverage commands are not being logged on windows
Reviewers: nate, max, schrockn, prha
Reviewed By: schrockn
Differential Revision: https://dagster.phacility.com/D5241