Add make target to workflow

Author: nhudson

.github/workflows/dashboards.yml

@@ -29,5 +29,5 @@ jobs:
       run: |
         go install github.com/monitoring-mixins/mixtool/cmd/mixtool@latest && \
         go install github.com/google/go-jsonnet/cmd/jsonnetfmt@latest && \
-        ./scripts/sync-mixins.sh && \
+        make sync-mixins && \
         git diff --exit-code