Use a HOSTNAME environment variable instead of socket.getfqdn() in ResultLog

[hroncok]

This makes it possible to prevent an undesired DNS query.

Fixes #1615

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Not quite sure where to document this behavior.

[hroncok]

Weird that the tests passed here, but when I try to package 3.17.0, I get:

==================================== ERRORS ====================================
___________________ ERROR at teardown of test_pre_set_header ___________________
[gw5] linux -- Python 3.9.0 /usr/bin/python3

    @pytest.fixture(autouse=True)
    def check_os_environ_stable():
        old = os.environ.copy()
    
        to_clean = {
            k: os.environ.pop(k, None)
            for k in {
                PARALLEL_ENV_VAR_KEY_PRIVATE,
                PARALLEL_ENV_VAR_KEY_PUBLIC,
                str("TOX_WORK_DIR"),
                str("PYTHONPATH"),
            }
        }
    
        yield
    
        for key, value in to_clean.items():
            if value is not None:
                os.environ[key] = value
    
        new = os.environ
        extra = {k: new[k] for k in set(new) - set(old)}
        miss = {k: old[k] for k in set(old) - set(new)}
        diff = {
            "{} = {} vs {}".format(k, old[k], new[k])
            for k in set(old) & set(new)
            if old[k] != new[k] and not k.startswith("PYTEST_")
        }
        if extra or miss or diff:
            msg = "test changed environ"
            if extra:
                msg += " extra {}".format(extra)
            if miss:
                msg += " miss {}".format(miss)
            if diff:
                msg += " diff {}".format(diff)
>           pytest.fail(msg)
E           Failed: test changed environ miss {'HOSTNAME': ''}

diff       = set()
extra      = {}
key        = 'TOX_PARALLEL_ENV'
miss       = {'HOSTNAME': ''}
msg        = "test changed environ miss {'HOSTNAME': ''}"
new        = environ({'PATH': '/builddir/build/BUILDROOT/python-tox-3.17.0-1.fc33.x86_64/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr...64/python3.9/site-packages:/builddir/build/BUILDROOT/python-tox-3.17.0-1.fc33.x86_64/usr/lib/python3.9/site-packages'})
old        = {'CFLAGS': '-O2 -fexceptions -g -grecord-gcc-switches -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,...-fstack-clash-protection -fcf-protection', 'CONFIG_SITE': 'NONE', 'HISTCONTROL': 'ignoredups', 'HISTSIZE': '1000', ...}
to_clean   = {'PYTHONPATH': '/builddir/build/BUILDROOT/python-tox-3.17.0-1.fc33.x86_64/usr/lib64/python3.9/site-packages:/builddir/...c33.x86_64/usr/lib/python3.9/site-packages', 'TOX_PARALLEL_ENV': None, 'TOX_WORK_DIR': None, '_TOX_PARALLEL_ENV': None}
value      = None

../../BUILDROOT/python-tox-3.17.0-1.fc33.x86_64/usr/lib/python3.9/site-packages/tox/_pytestplugin.py:100: Failed
=========================== short test summary info ============================
ERROR tests/unit/test_result.py::test_pre_set_header - Failed: test changed e...
======== 564 passed, 5 skipped, 1 warning, 1 error in 101.90s (0:01:41) ========

[hroncok]

Right, because when the HOSTNAME is '', it doesn't work properly.

[hroncok]

Followup: #1618