Fix VIRTUALENV_PYTHON environment lookup

The previous change which introduced the fallback discovery (pypa#1995)
broke this behaviour.

src/virtualenv/discovery/builtin.py

@@ -25,6 +25,7 @@ def add_parser_arguments(cls, parser):
             "--python",
             dest="python",
             metavar="py",
+            type=str,
             action="append",
             default=[],
             help="interpreter based on what to create environment (path/identifier) "

tests/unit/config/test_env_var.py

@@ -4,6 +4,7 @@
 
 import pytest
 
+from virtualenv.config.cli.parser import VirtualEnvOptions
 from virtualenv.config.ini import IniConfig
 from virtualenv.run import session_via_cli
 from virtualenv.util.path import Path
@@ -31,6 +32,13 @@ def test_value_bad(monkeypatch, caplog, empty_conf):
     assert "invalid literal" in caplog.messages[0]
 
 
+def test_python_via_env_var(monkeypatch):
+    options = VirtualEnvOptions()
+    monkeypatch.setenv(str("VIRTUALENV_PYTHON"), str("python3"))
+    session_via_cli(["venv"], options=options)
+    assert options.python == ["python3"]
+
+
 def test_extra_search_dir_via_env_var(tmp_path, monkeypatch):
     monkeypatch.chdir(tmp_path)
     value = "a{}0{}b{}c".format(os.linesep, os.linesep, os.pathsep)