presets: disable dnf-makecache.timer

That unit is enabled by default, but isn't actually activated on
OSTree systems. Outright disable it because otherwise it pulls in
`network-online.target` which we require to not be activated by default.

overlay.d/05rhcos/usr/lib/systemd/system-preset/43-manifest-rhcos.preset

@@ -23,3 +23,7 @@ enable console-login-helper-messages-issuegen.path
 # Enable nmstate. We can drop this when it is in
 # /usr/lib/systemd/system-preset/90-default.preset
 enable nmstate.service
+
+# This unit is not activated on OSTree systems, but it still pulls in
+# `network-online.target`. Explicitly disable it.
+disable dnf-makecache.timer