Use an instance attribute

As far as I can tell, during runtime there's only one EnvSelector
instance, so there's no danger of repeatedly warning about the same
skipped environment when .iter() gets called several times.

Fixes failing unit tests.  (Oops.  I only ran each test in isolation,
because the entire test suite is a bit slow.)

src/tox/session/env_select.py

@@ -108,13 +108,14 @@ class _ToxEnvInfo:
 
 class EnvSelector:
 
-    _warned_about: set[str] = set()  #: shared set of skipped environments that were already warned about
+    _warned_about: set[str]  #: shared set of skipped environments that were already warned about
 
     def __init__(self, state: State) -> None:
         # needs core to load the default tox environment list
         # to load the package environments of a run environments we need the run environment builder
         # to load labels we need core + the run environment
         self.on_empty_fallback_py = True
+        self._warned_about = set()
         self._state = state
         self._cli_envs: CliEnv | None = getattr(self._state.conf.options, "env", None)
         self._defined_envs_: None | dict[str, _ToxEnvInfo] = None