tox utility environments are constrainted to only run on Linux

[willkg]

We have four tox environments that I'm calling "utility environments":

• py39-docs
• py39-format-check
• py39-lint
• py39-vendorverify

In CI, we want these environments to only run on the Linux platform. In 4d6eae5a97ed7d1420eedf0a4bb3671afbf28fb1, I implemented that by constraining these four environments using platform = linux in the tox.ini file. One consequence of this is that a developer who is trying to run these on their local machine on macOS or Windows can no longer run them.

We need to figure out a way to constrain those tox environments such that they run iff:

1. in CI and platform = linux
2. not in CI

[willkg]

I'm not going to fix this.