Passing environment variables to conditionally formatted strings seems broken

## Issue



I'm not certain if this is a bug, or I'm not applying the new configuration logic properly. I would like to be able to conditionally pass environment variables to strings within the `deps` line.

My understanding of the documentation suggests that this is supported in more recent versions (https://tox.wiki/en/latest/reference/config.html#generative-environment-list), but it appears that when an environment variable is substituted within the string, it gets formatted into a list? i.e.

```toml
deps = [
  {replace = "if", condition = "env.ESMF_VERSION", then = ['git+https://github.com/esmf-org/esmf.git@v{env:ESMF_VERSION}#subdirectory=src/addon/esmpy']},
  {replace = "if", condition = "env.XCLIM_VERSION", then = ['xclim=={env:XCLIM_VERSION}']},
]
```
Effectively becomes:
```toml
deps = [
  ['git+https://github.com/esmf-org/esmf.git@v1.0.0#subdirectory=src/addon/esmpy'],
  ['xclim==0.59.0']
]
```
The tox config is otherwise consistent, and no issues arise unless the environment variables are set.

## Environment

Provide at least:

- OS: Linux

<details open>
<summary>Output of <code>pip list</code> of the host Python, where <code>tox</code> is installed</summary>

```console
tox                           4.46.3
```

</details>

## Output of running tox

<details open>
<summary>Output of <code>tox -rvv</code></summary>

```console
$ export ESMF_VERSION=1.0.0
$ export XCLIM_VERSION=0.59.0
$ tox -e py3.13   
py3.13: internal error
Traceback (most recent call last):
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/session/cmd/run/single.py", line 51, in _evaluate
    tox_env.setup()
    ~~~~~~~~~~~~~^^
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/tox_env/api.py", line 299, in setup
    self._setup_env()
    ~~~~~~~~~~~~~~~^^
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/tox_env/python/runner.py", line 152, in _setup_env
    self._install_deps()
    ~~~~~~~~~~~~~~~~~~^^
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/tox_env/python/runner.py", line 156, in _install_deps
    requirements_file: PythonDeps = self.conf["deps"]
                                    ~~~~~~~~~^^^^^^^^
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/config/sets.py", line 150, in __getitem__
    return self.load(item)
           ~~~~~~~~~^^^^^^
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/config/sets.py", line 162, in load
    return config_definition.__call__(self._conf, self.loaders, ConfigLoadArgs(chain, self.name, self.env_name))  # noqa: PLC2801
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/config/of_type.py", line 113, in __call__
    value = loader.load(primary_key, self.of_type, self.factory, conf, args, all_keys=alias_keys)
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/config/loader/api.py", line 153, in load
    converted = self.build(alias, of_type, factory, conf, raw, args)
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/config/loader/toml/__init__.py", line 75, in build
    result = self.to(exploded, of_type, factory)
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/config/loader/convert.py", line 56, in to
    return factory(raw)
  File "/home/tjs/mambaforge/envs/xscen/lib/python3.13/site-packages/tox/tox_env/python/pip/req_file.py", line 145, in factory
    raise TypeError(raw)
TypeError: [['git+https://github.com/esmf-org/esmf.git@v1.0.0#subdirectory=src/addon/esmpy'], ['xclim==0.59.0']]
  py3.13: FAIL code 2 (0.04 seconds)
  evaluation failed :( (0.07 seconds)
```

</details>

## Minimal example



```toml
deps = [
  {replace = "if", condition = "env.ESMF_VERSION", then = ['git+https://github.com/esmf-org/esmf.git@v{env:ESMF_VERSION}#subdirectory=src/addon/esmpy']},
  {replace = "if", condition = "env.XCLIM_VERSION", then = ['xclim=={env:XCLIM_VERSION}']},
]
```

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Uh oh!

Passing environment variables to conditionally formatted strings seems broken #3831

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Uh oh!

Passing environment variables to conditionally formatted strings seems broken #3831

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Issue

Environment

Output of running tox

Minimal example

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