Fix error 'Field does not exist: jaeger_mixin' in tk show

[benjaminhuo]

Signed-off-by: Benjamin benjamin@yunify.com

What this PR does / why we need it:
Got error like below when running tk show

tk show environments/loki/
evaluating jsonnet: RUNTIME ERROR: Field does not exist: jaeger_mixin
	/root/ben/loki/config/vendor/loki/query-frontend.libsonnet:16:5-19	object <anonymous>
	/root/ben/loki/config/vendor/loki/query-frontend.libsonnet:21:42-68	thunk from <thunk from <object <anonymous>>>
	<std>:237:55-61	thunk from <function <anonymous>>
	/root/ben/loki/config/vendor/ksonnet-util/kausal.libsonnet:317:27-28	function <addMount>
	<std>:237:50-62	function <anonymous>
		
	During manifestation	

[claassistantio]

All committers have signed the CLA.

[cyriltovena]

LGTM, sorry for that :) it was included on my usage.