Ensure tox_extend_envs list can be read twice

Previously (PR #3591) a new hook point was added. The tests checked
that it works well with the `tox config` and `tox list` commands.
However, `tox run` turned out to have a problem — it would complain
that there's no such env, when invoked:

```console
ROOT: 170 E HandledError| provided environments not found in configuration file:
pip-compile-tox-env-lock [tox/run.py:23]
```

Turned out, this was because the config object is being interated twice
in some subcommands. This in turn iterates over the discovered
additional ephemeral environments list object. The implementation passes
an iterator into it and so when it's first accessed, it's exhausted and
the second attempt does not give the same envs, causing inconsistency.

The patch solves this by using `itertools.tee()`, making sure that the
underlying iterable is always cached and it's possible to repeat
iteration as many times as possible without loosing the data in the
process.

src/tox/config/main.py

@@ -2,7 +2,7 @@
 
 import os
 from collections import OrderedDict, defaultdict
-from itertools import chain
+from itertools import chain, tee
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Iterable, Iterator, Sequence, TypeVar
 
@@ -81,7 +81,7 @@ def src_path(self) -> Path:
 
     def __iter__(self) -> Iterator[str]:
         """:return: an iterator that goes through existing environments"""
-        return chain(self._src.envs(self.core), self._extra_envs)
+        return chain(self._src.envs(self.core), chain.from_iterable(tee(self._extra_envs)))
 
     def sections(self) -> Iterator[Section]:
         yield from self._src.sections()

src/tox/session/state.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import sys
-from itertools import chain
+from itertools import chain, tee
 from typing import TYPE_CHECKING, Sequence
 
 from tox.config.main import Config
@@ -20,7 +20,7 @@ class State:
     """Runtime state holder."""
 
     def __init__(self, options: Options, args: Sequence[str]) -> None:
-        extended_envs = chain.from_iterable(MANAGER.tox_extend_envs())
+        (extended_envs,) = tee(chain.from_iterable(MANAGER.tox_extend_envs()), 1)
         self.conf = Config.make(options.parsed, options.pos_args, options.source, extended_envs)
         self.conf.core.add_constant(
             keys=["on_platform"],

tests/plugin/test_inline.py

@@ -43,6 +43,7 @@ def tox_extend_envs() -> tuple[str]:
         def tox_add_core_config(core_conf: ConfigSet, state: State) -> None:  # noqa: ARG001
             in_memory_config_loader = MemoryLoader(
                 base=["sentinel-base"],
+                commands_pre=["sentinel-cmd"],
                 description="sentinel-description",
             )
             state.conf.memory_seed_loaders[env_name].append(
@@ -59,3 +60,10 @@ def tox_add_core_config(core_conf: ConfigSet, state: State) -> None:  # noqa: AR
     tox_config_result = project.run("config", "-e", "sentinel-env-name", "-qq")
     tox_config_result.assert_success()
     assert "base = sentinel-base" in tox_config_result.out
+
+    tox_run_result = project.run("run", "-e", "sentinel-env-name", "-q")
+    tox_run_result.assert_failed()
+    expected_cmd_lookup_error_txt = (
+        "sentinel-env-name: Exception running subprocess [Errno 2] No such file or directory: 'sentinel-cmd'\n"
+    )
+    assert expected_cmd_lookup_error_txt in tox_run_result.out