Check environment variable PIPENV_CUSTOM_VENV_NAME

Resolves #1226

Author: otherjake

pipenv/project.py

@@ -365,6 +365,9 @@ def get_name(name, location):
 
     @property
     def virtualenv_name(self) -> str:
+        custom_name = os.getenv("PIPENV_CUSTOM_VENV_NAME")
+        if custom_name:
+            return custom_name
         sanitized, encoded_hash = self._get_virtualenv_hash(self.name)
         suffix = ""
         if self.s.PIPENV_PYTHON: