Set Grafana dashboard variable from Prometheus scalar

I need to use the number of CPU cores in multiple calculations on a Grafana dashboard and was hoping to perform the query once and store in a variable. I can't find a way to return the data in a format that Grafana will accept.

Grafana Error:

Template variables could not be initialized: parse error at char 1: vector selector must contain label matchers or metric name

I have tried (PromQL):

count(node_cpu{instance="$host",mode="system"})

Returns:

Element {}, Value 2

Also (PromQL):

scalar(count(node_cpu{instance="$host",mode="system"}))

Returns:

Element scalar, Value 2

Using Grafana's query_result function like so:

query_result(count(node_cpu{instance="$host",mode="system"}))

I get the following values in the preview:

{} 2 1521669355000

The 2 in the middle is the one I want.

Any ideas how to make this work?