When I try to run ./mach wpt foo --product chrome, I see this error:

FileNotFoundError: [Errno 2] No such file or directory: '<dist-dir>/_virtualenvs/common/bin/python'
  File "<src-dir>/testing/web-platform/mach_commands.py", line 531, in run_wpt
    return run_web_platform_tests(command_context, **params)
  File "<src-dir>/testing/web-platform/mach_commands.py", line 520, in run_web_platform_tests
    return wpt_runner.run(logger, **params)
  File "<src-dir>/testing/web-platform/mach_commands_base.py", line 57, in run
    kwargs = self.setup.kwargs_wptrun(kwargs)
  File "<src-dir>/testing/web-platform/mach_commands.py", line 188, in kwargs_wptrun
    self.virtualenv_manager.install_pip_requirements(
  File "<src-dir>/python/mach/mach/site.py", line 638, in install_pip_requirements
    install_result = self._virtualenv.pip_install(
  File "<src-dir>/python/mach/mach/site.py", line 846, in pip_install
    return subprocess.run(
  File "/usr/lib/python3.10/subprocess.py", line 501, in run
    with Popen(*popenargs, **kwargs) as process:
  File "<src-dir>/third_party/python/sentry_sdk/sentry_sdk/integrations/stdlib.py", line 190, in sentry_patched_popen_init
    rv = old_popen_init(self, *a, **kw)  # type: ignore
  File "/usr/lib/python3.10/subprocess.py", line 966, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.10/subprocess.py", line 1842, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)

I'm back from PTO today and I upgraded my Linux environment from Ubuntu 21.10 to 22.04. So, I'm not sure whether this is a bug in our tree or not. (Of course, I rebuilt Firefox after deleting the dist-dir, and tried ./mach bootstrap too.) However, on Windows, I also see similar stack (On Windows, it's not printed that which file was not found).

And there is only build in <dist-dir>/_virtualenvs, i.e., no common directory.

Oh, the common directory appears today's mozilla-central and it works for me.

It really only works by accident; it depends if you ran another mach command that's using the common virtualenv. However I'd just made a patch for this issue and it should be fixed properly as soon as that gets review.