Fix buggy implementation of --system check

Signed-off-by: Dan Ryan <dan@danryan.co>

pipenv/core.py

@@ -69,6 +69,7 @@ from .environments import (
     PIPENV_DOTENV_LOCATION,
     PIPENV_SHELL,
     PIPENV_PYTHON,
+    PIPENV_VIRTUALENV,
 )
 
 # Backport required for earlier versions of Python.
@@ -247,12 +248,12 @@ def import_from_code(path='.'):
 
 def ensure_pipfile(validate=True, skip_requirements=False, system=False):
     """Creates a Pipfile for the project, if it doesn't exist."""
-    global USING_DEFAULT_PYTHON
+    global USING_DEFAULT_PYTHON, PIPENV_VIRTUALENV
     # Assert Pipfile exists.
     python = which('python') if not (USING_DEFAULT_PYTHON or system) else None
     if project.pipfile_is_empty:
         # Show an error message and exit if system is passed and no pipfile exists
-        if system:
+        if system and not PIPENV_VIRTUALENV:
             click.echo(
                 '{0}: --system is intended to be used for pre-existing Pipfile '
                 'installation, not installation of specific packages. Aborting.'.format(
@@ -1756,7 +1757,7 @@ def do_install(
         keep_outdated = project.settings.get('keep_outdated')
     remote = requirements and is_valid_url(requirements)
     # Warn and exit if --system is used without a pipfile.
-    if system and package_name:
+    if system and package_name and not PIPENV_VIRTUALENV:
         click.echo(
             '{0}: --system is intended to be used for Pipfile installation, '
             'not installation of specific packages. Aborting.'.format(