unable to use substitution when using file|

[piotr-kubiak]

With tox.ini like this:

[tox]
env_list = example1

[testenv]
set_env =
    FOO=1
allowlist_externals = bash
commands =
    bash -c 'env | grep "FOO"'

[testenv:example1]
set_env =
    {[testenv]set_env}

Everything works as expected. However, when I replace FOO=1 with file|.env in set_env:

set_env =
    file|.env

then I get an error: ValueError: invalid line 'file|.env' in set_env.

Expected: being able to use substitution when using file|.