pytest-cov: Failed to setup subprocess coverage if os.getcwd() fails

Upstream bug: https://github.com/pytest-dev/pytest-cov/issues/541

Following sample test case causes pytest-cov to fail:

```
import os
import subprocess
import sys
import tempfile
import textwrap
import unittest

class Test(unittest.TestCase):
    maxDiff = None

    def test_deleted_working_directory(self):
        """Relative Python script from deleted working directory."""
        orig_cwd = os.getcwd()
        try:
            with tempfile.TemporaryDirectory() as tmpdir:
                deleted_dir = os.path.join(tmpdir, "gone")
                os.mkdir(deleted_dir)
                os.chdir(deleted_dir)
                os.rmdir(deleted_dir)

                with open(os.path.join(tmpdir, "script.py"), "w") as script:
                    script.write("raise ValueError()\n")

                env = os.environ.copy()
                env["PYTHONPATH"] = orig_cwd
                process = subprocess.run(
                    [sys.executable, "../script.py"],
                    env=env,
                    stdout=subprocess.PIPE,
                    stderr=subprocess.PIPE,
                )
        finally:
            os.chdir(orig_cwd)

        print(process.stderr.decode())
        self.assertEqual(
            process.stderr.decode(),
            textwrap.dedent(
                """\
                Traceback (most recent call last):
                  File "../script.py", line 1, in <module>
                    raise ValueError()
                ValueError
                """
            ),
        )
        self.assertEqual(process.stdout.decode(), "")
        self.assertEqual(process.returncode, 1)
```

Run this script with `python3 -m pytest --cov=. test.py` and stderr will contain following stack trace:

```
Error processing line 1 of /usr/lib/python3/dist-packages/init_cov_core.pth:

  Traceback (most recent call last):
    File "/usr/lib/python3.10/site.py", line 192, in addpackage
      exec(line)
    File "<string>", line 1, in <module>
    File "/usr/lib/python3/dist-packages/cov_core_init.py", line 20, in <module>
      import cov_core # noqa: register multiprocessing handler
    File "/usr/lib/python3/dist-packages/cov_core.py", line 5, in <module>
      import coverage
    File "/usr/lib/python3/dist-packages/coverage/__init__.py", line 15, in <module>
      from coverage.control import Coverage, process_startup
    File "/usr/lib/python3/dist-packages/coverage/control.py", line 17, in <module>
      from coverage.annotate import AnnotateReporter
    File "/usr/lib/python3/dist-packages/coverage/annotate.py", line 9, in <module>
      from coverage.files import flat_rootname
    File "/usr/lib/python3/dist-packages/coverage/files.py", line 166, in <module>
      set_relative_directory()
    File "/usr/lib/python3/dist-packages/coverage/files.py", line 28, in set_relative_directory
      RELATIVE_DIR = os.path.normcase(abs_file(os.curdir) + os.sep)
    File "/usr/lib/python3/dist-packages/coverage/files.py", line 149, in abs_file
      path = os.path.realpath(path)
    File "/usr/lib/python3.10/posixpath.py", line 396, in realpath
      return abspath(path)
    File "/usr/lib/python3.10/posixpath.py", line 383, in abspath
      cwd = os.getcwd()
  FileNotFoundError: [Errno 2] No such file or directory

Remainder of file ignored
pytest-cov: Failed to setup subprocess coverage.
```