Under the Metrics tab, add new metric that will be hidden in the chart and is used for alerting only. Duplicate the query and remove all template variables (i.e. $somevar) from it. Replace the template variable with a hard-coded value you want to create alert for. Hide the metric by clicking on the “eye” icon.
Source: https://community.grafana.com/t/template-variables-are-not-supported-in-alert-queries-while-setting-up-alert/2514/8