gentoo.py: Unlock >=app-emulation/cloud-init-23.4 with Python 3.12

Symptom was:
>  !!! The ebuild selected to satisfy "app-emulation/cloud-init" has unmet requirements.
> - app-emulation/cloud-init-22.4::gentoo USE="(-selinux) -test" ABI_X86="(64)" PYTHON_TARGETS="-python3_10 -python3_11"
>
>   The following REQUIRED_USE flag constraints are unsatisfied:
>     any-of ( python_targets_python3_10 python_targets_python3_11 )

image_bootstrap/distros/gentoo.py

@@ -274,6 +274,8 @@ def install_cloud_init_and_friends(self):
         #      instead of dev-lang/rust (which asked for "11520 MiB disk space")
         self._add_package_mask('dev-lang/rust')
 
+        self._set_package_keywords('app-emulation/cloud-init', f'~{_HOST_ARCH}')
+
         self._install_package_atoms(['app-emulation/cloud-init', 'net-misc/openssh'])
         self.disable_cloud_init_syslog_fix_perms()
         self.install_growpart()