Fix undefined variable

[chzhyang]

NameError: name 'root' is not defined: parameter root of VenvPipInstallFailedError() is undefined, it should be self.root.

NameError before changed root to self.root

Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/__main__.py", line 2, in <module>
    pyperformance.cli.main()
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/cli.py", line 317, in main
    _main()
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/cli.py", line 301, in _main
    cmd_run(options, benchmarks)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/commands.py", line 180, in cmd_run
    suite, errors = run_benchmarks(benchmarks, executable, options)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/run.py", line 61, in run_benchmarks
    common = VenvForBenchmarks.ensure(
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/venv.py", line 193, in ensure
    return cls.create(
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/venv.py", line 162, in create
    self.ensure_pip(upgrade=upgrade)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/_venv.py", line 220, in ensure_pip
    raise VenvPipInstallFailedError(root, ec)
NameError: name 'root' is not defined

After changed root to self.root:

Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/__main__.py", line 2, in <module>
    pyperformance.cli.main()
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/cli.py", line 317, in main
    _main()
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/cli.py", line 301, in _main
    cmd_run(options, benchmarks)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/commands.py", line 180, in cmd_run
    suite, errors = run_benchmarks(benchmarks, executable, options)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/run.py", line 61, in run_benchmarks
    common = VenvForBenchmarks.ensure(
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/venv.py", line 193, in ensure
    return cls.create(
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/venv.py", line 162, in create
    self.ensure_pip(upgrade=upgrade)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/pyperformance/_venv.py", line 220, in ensure_pip
    raise VenvPipInstallFailedError(self.root, ec)
pyperformance._venv.VenvPipInstallFailedError: failed to install pip in venv /home/ubuntu/pyperformance/venv/cpython3.11-f90a0555cbd0-compat-07e19f9c3993