Fix accidental changes to PYTHONPATH in os.environ

When generating an environment dict for subprocess calls to pip,
os.environ was accidentally modified despite efforts to copy it
and modify only the copy, as copy.copy(os.environ) is not enough.

src/zc/buildout/easy_install.py

@@ -1696,8 +1696,8 @@ def call_pip_install(spec, dest):
         else:
             args.append('--no-python-version-warning')
 
-    env = copy.copy(os.environ)
-    python_path = copy.copy(pip_path)
+    env = os.environ.copy()
+    python_path = pip_path[:]
     python_path.append(env.get('PYTHONPATH', ''))
     env['PYTHONPATH'] = os.pathsep.join(python_path)