To-upstream: Symlink in /etc/mkosi-manifest

Will upstream after/if this is merged:
systemd/mkosi#3997

Author: jcgl17

mkosi.extra/usr/lib/tmpfiles.d/etc.conf

@@ -3,6 +3,7 @@
 # This overrides the same file from systemd since we want to symlink everything
 # into /etc instead of copying so updates to /usr propagate properly.
 L /etc/os-release - - - - ../usr/lib/os-release
+L /etc/mkosi-manifest - - - - ../usr/lib/mkosi-manifest
 L+ /etc/mtab - - - - ../proc/self/mounts
 # Contains the default systemd locale
 L /etc/locale.conf