Pin RHEL 9 builds to 9.1 (#999)

RHEL 9.2 has been released, but there are packages in EPEL which need to
be rebuilt before we can make the change.

For now, let's pin to RHEL 9.1 to unblock builds.

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

@@ -41,6 +41,15 @@ config_opts['macros']['%_without_weak_deps'] = '1'
 config_opts['chroot_setup_cmd'] += ' gcc-c++ make'
 @[end if]@
 
+@[if os_name == 'rhel' and os_code_name == '9']@
+# Pin RHEL 9 to 9.1 everywhere except EPEL
+import re
+config_opts[f'{config_opts.package_manager}.conf'] = re.sub(
+ r'(?<!(epel|EPEL)-)\$releasever',
+ '9.1',
+ config_opts[f'{config_opts.package_manager}.conf'])
+@[end if]@
+
 config_opts[f'{config_opts.package_manager}.conf'] += """
 @[for i, url in enumerate(distribution_repository_urls)]@
 [ros-buildfarm-@(i)]