> Because sys.executable (argv[0] in fact) is not filled when you call
> multiprocess vvia the -j option.

sys.executable is set perfectly well when running regrtest in
multiprocess mode, otherwise many other tests would fail.

Try:
$ grep "sys.executable" Lib/test/*
Lib/test/regrtest.py:585:        base_cmd = [sys.executable] + opt_args + ['-m', 'test.regrtest']
Lib/test/script_helper.py:20:    cmd_line = [sys.executable]
Lib/test/script_helper.py:60:    cmd_line = [sys.executable, '-E']
Lib/test/test_base64.py:231:        args = (sys.executable, '-m', 'base64') + args
Lib/test/test_capi.py:42:        p = subprocess.Popen([sys.executable, "-c",
[etc.]

Again, it seems sysconfig simply has a bug which you can reproduce on
the command line:

$ pwd
/home/antoine/cpython/default/build
$ ../python -c "import sys, sysconfig; print(sys.executable,
sysconfig.get_config_var('srcdir'))"
/home/antoine/cpython/default/build/../python /home/antoine/cpython/default/build