Restore PM-specific option specification in mock config (#1074)

It appears that we actually WANT this configuration key to change with
the package manager, and this will be required as mock now supports
multiple DNF tools.

This is all a bit of a mess right now. There isn't a straightforward way
to determine which package manager we should be using, and mock is in
the process of transitioning the default PM to dnf5, where it was
previously unversioned dnf (which means dnf4).

This reverts part of ed19495

ros_buildfarm/templates/release/rpm/mock_config.cfg.em

@@ -1,5 +1,13 @@
 include('/etc/mock/default.cfg')
 
+def _expanded(k, opts):
+    """Get a config value and ensure it has been expanded."""
+    exp, opts['__jinja_expand'] = opts.get('__jinja_expand', False), True
+    try:
+        return opts[k]
+    finally:
+        opts['__jinja_expand'] = exp
+
 # Change the root name since we're modifying the chroot
 config_opts['root'] += '-ros-buildfarm'
 
@@ -10,7 +18,13 @@ config_opts['use_bootstrap'] = False
 config_opts['chroot_setup_cmd'] += ' python3-rpm-macros'
 
 # Install weak dependencies to get group members
-config_opts[f'dnf_builddep_opts'] = config_opts.get(f'dnf_builddep_opts', []) + ['--setopt=install_weak_deps=True']
+package_manager = _expanded('package_manager', config_opts)
+additional_opts = ['--setopt=install_weak_deps=True']
+config_opts[f'{package_manager}_builddep_opts'] = config_opts.get(f'{package_manager}_builddep_opts', []) + additional_opts
+
+# Deal with dnf -> dnf4 transition - see rpm-software-management/mock#1496
+if package_manager == 'dnf':
+    config_opts[f'dnf4_builddep_opts'] = config_opts.get(f'dnf4_builddep_opts', []) + additional_opts
 
 @[if env_vars]@
 # Set environment vars from the build config