pytest mode: Don't push git snapshot during pytest collection
pytest mode: Don't push git snapshot during pytest collection

VS Code's "Discover Tests" command runs pytest with `--collect-only`.
This was taking a long time because we were pushing a git snapshot.

It would also fail intermittently due to a race with our "Run On Save"
configuration which also runs `stbt_rig snapshot`: The git push would
fail due to the upstream changing. This would cause VS Code's "Discover
Tests" to fail intermittently with an error message.

stb-tester/stbt-rig