$ ls -al $(which python2)
lrwxr-xr-x  1 brettmz  admin  37 Nov 15 12:04 /usr/local/bin/python2 -> ../Cellar/python@2/2.7.17/bin/python2

$ virtualenv --python /usr/local/bin/python2 py2env
Traceback (most recent call last):
  File "/Users/brettmz/Library/Python/3.7/bin/virtualenv", line 8, in <module>
    sys.exit(run())
  File "/Users/brettmz/Library/Python/3.7/lib/python/site-packages/virtualenv/__main__.py", line 16, in run
    run_via_cli(args)
  File "/Users/brettmz/Library/Python/3.7/lib/python/site-packages/virtualenv/run/__init__.py", line 21, in run_via_cli
    session = session_via_cli(args)
  File "/Users/brettmz/Library/Python/3.7/lib/python/site-packages/virtualenv/run/__init__.py", line 33, in session_via_cli
    raise RuntimeError("failed to find interpreter for {}".format(discover))
RuntimeError: failed to find interpreter for Builtin discover of python_spec='/usr/local/bin/python2'