fix bug - root is not defined (#270)

Author: chzhyang

pyperformance/_venv.py

@@ -215,9 +215,9 @@ def ensure_pip(self, downloaddir=None, *, installer=True, upgrade=True):
             upgrade=upgrade,
         )
         if ec != 0:
-            raise VenvPipInstallFailedError(root, ec)
+            raise VenvPipInstallFailedError(self.root, ec)
         elif not _pip.is_pip_installed(self.python, env=self._env):
-            raise VenvPipInstallFailedError(root, 0, "pip doesn't work")
+            raise VenvPipInstallFailedError(self.root, 0, "pip doesn't work")
 
         if installer:
             # Upgrade installer dependencies (setuptools, ...)