Do not fail (not just) hermetic builds for missing resolv.conf

First, Mock cannot guarantee that the host environment provides a
resolv.conf file.  We should simply warn the user instead of triggering
a hard failure.

Second, the check for "simple" isolation was incomplete.  We need to
check for USE_NSPAWN, as it accurately reflects the isolation=auto case.

INFO: mock.py version 6.6 starting (python version = 3.14.2, NVR = mock-6.6-1.fc43), args: /usr/libexec/mock/mock --hermetic-build /buildroot/results/buildroot_lock.json /buildroot/results/buildroot_repo --spec /source/libecpg.spec --sources /source --resultdir /results
Traceback (most recent call last):
  File "/usr/libexec/mock/mock", line 1132, in <module>
    exitStatus = main()
  File "/usr/lib/python3.14/site-packages/mockbuild/trace_decorator.py", line 93, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 769, in main
    util.setup_host_resolv(bootstrap_buildroot_config)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/site-packages/mockbuild/trace_decorator.py", line 93, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.14/site-packages/mockbuild/util.py", line 890, in setup_host_resolv
    shutil.copyfile('/etc/resolv.conf', resolv_path)
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/shutil.py", line 313, in copyfile
    with open(src, 'rb') as fsrc:
         ~~~~^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/etc/resolv.conf'

Relates: https://bugzilla.redhat.com/show_bug.cgi?id=2433808
Closes: rpm-software-management#1697

Author: praiskup

mock/py/mockbuild/util.py

@@ -860,6 +860,7 @@ def clean_env():
 
 @traceLog()
 def setup_host_resolv(config_opts):
+    log = getLog()
     if not config_opts['use_host_resolv']:
         # If we don't copy host's resolv.conf, we at least want to resolve
         # our own hostname.  See commit 28027fc26d.
@@ -869,7 +870,7 @@ def setup_host_resolv(config_opts):
                 ::1       localhost localhost.localdomain localhost6 localhost6.localdomain6
                 ''')
 
-    if config_opts['isolation'] == 'simple':
+    if not USE_NSPAWN:
         # Not using nspawn -> don't touch /etc/resolv.conf; we already have
         # a valid file prepared by Buildroot._init() (if user requested).
         return
@@ -889,7 +890,10 @@ def setup_host_resolv(config_opts):
     os.chmod(resolv_path, 0o644)
 
     if config_opts['use_host_resolv']:
-        shutil.copyfile('/etc/resolv.conf', resolv_path)
+        try:
+            shutil.copyfile('/etc/resolv.conf', resolv_path)
+        except FileNotFoundError:
+            log.warning("Non-existing resolv.conf on host, using an empty one")
 
     config_opts['nspawn_args'] += ['--bind={0}:/etc/resolv.conf'.format(resolv_path)]