Resolve default config path after the user is known

The default config path was computed before -u was parsed, so
running as root without sudo/doas used '~None/...' even when the
user was given with -u. Expand '~' only after the user is detected.

simple_backup/simple_backup.py

@@ -652,9 +652,6 @@ def _detect_user() -> Optional[str]:
 
 
 def _parse_arguments() -> argparse.Namespace:
-    user = _detect_user()
-    homedir = os.path.expanduser(f'~{user}')
-
     parser = argparse.ArgumentParser(prog='simple_backup',
                                      description='Simple backup script written in Python that uses rsync to copy files',
                                      epilog='See simple_backup(1) manpage for full documentation',
@@ -662,7 +659,7 @@ def _parse_arguments() -> argparse.Namespace:
 
     parser.add_argument('-v', '--verbose', action='store_true',
                         help='More verbose output')
-    parser.add_argument('-c', '--config', default=f'{homedir}/.config/simple_backup/simple_backup.conf',
+    parser.add_argument('-c', '--config', default='~/.config/simple_backup/simple_backup.conf',
                         help='Specify location of configuration file')
     parser.add_argument('-i', '--inputs', nargs='+',
                         help='Paths/files to backup')
@@ -896,8 +893,16 @@ def simple_backup() -> int:
         except NameError:
             pass
 
+    config_file = args.config
+
+    # Expand '~' to the home of the user running the backup, not root's
+    if config_file.startswith('~/') and user is not None:
+        config_file = f'~{user}{config_file[1:]}'
+
+    config_file = os.path.expanduser(config_file)
+
     try:
-        config_args = _read_config(args.config, user)
+        config_args = _read_config(config_file, user)
     except (configparser.NoSectionError, configparser.NoOptionError):
         logger.critical('Bad configuration file')