fix(parser): Fix --discover parsed incorrectly from env

The flag was missing the type specification (`of_type`) and thus
parse.get_env_var did not execute the branch for list, but rather for
single variables.
The fallback in Converter lead to a call to `list("abc")`, which
resulted int a list of characters (["a", "b", "c"]).

src/tox/config/cli/parser.py

@@ -232,6 +232,7 @@ def __call__(
             dest="discover",
             nargs="+",
             metavar="path",
+            of_type=list[str],
             help="for Python discovery first try the Python executables under these paths",
             default=[],
         )

tests/config/cli/test_cli_env_var.py

@@ -84,6 +84,7 @@ def test_env_var_exhaustive_parallel_values(
     monkeypatch.setenv("TOX_PARALLEL", "3")
     monkeypatch.setenv("TOX_PARALLEL_LIVE", "no")
     monkeypatch.setenv("TOX_OVERRIDE", "a=b;c=d")
+    monkeypatch.setenv("TOX_DISCOVER", "/foo/bar;/bar/baz;/baz/foo")
 
     options = get_options()
     assert vars(options.parsed) == {
@@ -93,7 +94,7 @@ def test_env_var_exhaustive_parallel_values(
         "default_runner": "virtualenv",
         "develop": False,
         "devenv_path": None,
-        "discover": [],
+        "discover": ["/foo/bar", "/bar/baz", "/baz/foo"],
         "env": CliEnv(["py37", "py36"]),
         "force_dep": [],
         "hash_seed": ANY,