Fix --site-packages flag is not recognized

`site_packages` argument in `cli` function always set to false because `--site-packages` flag was defined in global state (https://github.com/pypa/pipenv/blob/97657b2797b8a8f045776470d74add6048a95b67/pipenv/cli/options.py#L62). 

Fixes #3351

pipenv/cli/command.py

@@ -70,7 +70,6 @@ def cli(
     python=False,
     help=False,
     py=False,
-    site_packages=False,
     envs=False,
     man=False,
     completion=False,
@@ -198,7 +197,7 @@ def cli(
                 )
                 ctx.abort()
     # --two / --three was passed…
-    if (state.python or state.three is not None) or site_packages:
+    if (state.python or state.three is not None) or state.site_packages:
         ensure_project(
             three=state.three,
             python=state.python,