Treat "env changed" as a warning, not a failure

[vstinner]

No description provided.

[vstinner]

Example on AMD64 FreeBSD15 NoGIL PR: https://buildbot.python.org/#/builders/2369/builds/2

1 test failed:
    test.test_free_threading.test_capi
513 tests OK.
(..)
Result: FAILURE
*** Error code 2

[vstinner]

A few years ago, I was tracking buildbot failures more closely and I wanted be notified when anything bad happened. I added "env changed" to libregrtest when a test passed but the test modified a common resource, leaked a subprocess/thread, etc. I treated "env changed" so I would get an email notification for example.

Nowadays, we have a way larger set of buildbot workers. It became really hard to keep track of all of them.

I propose to treat "env changed" as a warning (orange), instead of a failure (red).

@encukou @zware: Are you ok with this change?