Show a better hint for broken PBS venv

[konstin]

For some reason, I'm encountering broken PBS venvs again (astral-sh/python-build-standalone#380). Instead of showing the inscrutable error to the user, we point them to the PBS bug and ask them to run uv venv.

Fixed a duplicate "exit code" in the first commit.

Before

$ uv sync
error: Querying Python at `/tmp/uv/tests/.tmpwXZRTI/temp/.venv/bin/python3` failed with exit status exit status: 1

[stderr]
Could not find platform independent libraries <prefix>
Could not find platform dependent libraries <exec_prefix>
Python path configuration:
  PYTHONHOME = (not set)
  PYTHONPATH = (not set)
  program name = '/tmp/uv/tests/.tmpwXZRTI/temp/.venv/bin/python3'
  isolated = 1
  environment = 0
  user site = 0
  safe_path = 1
  import site = 1
  is in build tree = 0
  stdlib dir = '/install/lib/python3.12'
  sys._base_executable = '/home/konsti/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/bin/python3.12'
  sys.base_prefix = '/install'
  sys.base_exec_prefix = '/install'
  sys.platlibdir = 'lib'
  sys.executable = '/tmp/uv/tests/.tmpwXZRTI/temp/.venv/bin/python3'
  sys.prefix = '/install'
  sys.exec_prefix = '/install'
  sys.path = [
    '/install/lib/python312.zip',
    '/install/lib/python3.12',
    '/install/lib/python3.12/lib-dynload',
  ]
Fatal Python error: init_fs_encoding: failed to get the Python codec of the filesystem encoding
Python runtime state: core initialized
ModuleNotFoundError: No module named 'encodings'

Current thread 0x000078e48b835740 (most recent call first):
  <no Python frame>

After

$ uv sync
  error: Can't use Python at `/tmp/uv/tests/.tmpwXZRTI/temp/.venv/bin/python3`
    Caused by: Python is missing PYTHONHOME. If you are using a managed Python interpreter, this is a known bug (https://github.com/astral-sh/python-build-standalone/issues/380). You can recreate the virtual environment with `uv venv`.

[konstin]

This isn't ideal but it's consistent with the existing logic.

[zanieb]

cc @geofft