env_select: ignore labels when provisioning

Do not attempt to set any test environments as active based on
user-supplied labels when tox will provision a new environment and
re-execute itself.

If any labeled environments depend on a future tox version or external
plugin, their configuration will be invalid and thus cannot be set
active.

fix #2916

Author: masenf

docs/changelog/2916.bugfix.rst

@@ -0,0 +1,3 @@
+Ignore labels when tox will provision a runtime environment (``.tox``) so that environment configurations which depend
+on provisioned plugins or specific tox versions are not accessed in the outer tox process where the configuration would
+be invalid - by :user:`masenf`.

src/tox/session/env_select.py

@@ -315,7 +315,8 @@ def _mark_active(self) -> None:
         if labels or factors:
             for env_info in self._defined_envs_.values():
                 env_info.is_active = False  # if any was selected reset
-            if labels:
+            # ignore labels when provisioning will occur
+            if labels and (self._provision is None or not self._provision[0]):
                 for label in labels:
                     for env_name in self._state.conf.core["labels"].get(label, []):
                         self._defined_envs_[env_name].is_active = True