Work around tox4 --installpkg compat issue

Ref: tox-dev/tox#2442

tox.ini

@@ -25,6 +25,10 @@ description =
   Build dists and put them into the dist{/} folder
 basepython = python3
 isolated_build = true
+# NOTE: `package_env = none` is needed so it's possible to use `--installpkg`
+# NOTE: with the main `testenv`.
+# Ref: https://github.com/tox-dev/tox/issues/2442
+package_env = ❌ DUMMY NON-EXISTENT ENV NAME ❌
 # `usedevelop = true` overrides `skip_install` instruction, it's unwanted
 usedevelop = false
 skip_install = true