Update giantswarm/helm-chart-testing Docker tag to v3.12.0 (#278)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.circleci/config.yml

@@ -4,7 +4,7 @@ orbs:
 jobs:
   template-chart:
     docker:
-      - image: giantswarm/helm-chart-testing:v3.11.0
+      - image: giantswarm/helm-chart-testing:v3.12.0
     steps:
       - checkout
       - run: cd helm/grafana && helm dep up