fix(parser): Fix --discover parsed incorrectly from env (#3274)

Author: mimre25

docs/changelog/3272.bugfix.rst

@@ -0,0 +1 @@
+Fix broken input parsing for ``--discover`` flag. - by :user:`mimre25`

docs/changelog/3274.doc.rst

@@ -0,0 +1 @@
+Rephrase ``--discover`` flag's description to avoid confusion between paths and executables. - by :user:`mimre25`

src/tox/config/cli/parser.py

@@ -232,7 +232,8 @@ def __call__(
             dest="discover",
             nargs="+",
             metavar="path",
-            help="for Python discovery first try the Python executables under these paths",
+            of_type=List[str],
+            help="for Python discovery first try these Python executables",
             default=[],
         )
         list_deps = sub_parser.add_mutually_exclusive_group()

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,