Running twister with code coverage causes an exception

[keith-zephyr]

Describe the bug
Running twister to generate coverage against a non-native board causes an exception.

To Reproduce
Steps to reproduce the behavior:

1. ./scripts/twister -T tests/arch/x86/nmi/ --coverage
2. Exception generated:

Traceback (most recent call last):
  File "/big_ssd/keithshort/zephyrproject/zephyr/./scripts/twister", line 214, in <module>
    ret = main(options)
          ^^^^^^^^^^^^^
  File "/big_ssd/keithshort/zephyrproject/zephyr/scripts/pylib/twister/twisterlib/twister_main.py", line 205, in main
    coverage_completed = run_coverage(tplan, options)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/big_ssd/keithshort/zephyrproject/zephyr/scripts/pylib/twister/twisterlib/coverage.py", line 351, in run_coverage
    coverage_completed = coverage_tool.generate(options.outdir)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/big_ssd/keithshort/zephyrproject/zephyr/scripts/pylib/twister/twisterlib/coverage.py", line 122, in generate
    ret = self._generate(outdir, coveragelog)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/big_ssd/keithshort/zephyrproject/zephyr/scripts/pylib/twister/twisterlib/coverage.py", line 282, in _generate
    subprocess.call(["gcovr", "-r", self.base_dir, "--gcov-executable",
  File "/usr/lib/python3.11/subprocess.py", line 389, in call
    with Popen(*popenargs, **kwargs) as p:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 1026, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.11/subprocess.py", line 1883, in _execute_child
    self.pid = _fork_exec(
               ^^^^^^^^^^^
TypeError: expected str, bytes or os.PathLike object, not NoneType