👷(ci) lint helmfile

Introduced by @rouja. Added a new linter to ensure helm and yaml
files can be properly parsed into templates.
ArgoCD can not break anymore.

.github/workflows/helmfile-linter.yml

@@ -0,0 +1,22 @@
+name: Helmfile lint
+run-name: Helmfile lint
+
+on:
+  pull_request:
+    branches:
+      - 'main'
+
+jobs:
+  helmfile-lint:
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/helmfile/helmfile:latest
+    steps:
+      -
+        uses: numerique-gouv/action-helmfile-lint@main
+        with:
+          app-id: ${{ secrets.APP_ID }}
+          age-key: ${{ secrets.SOPS_PRIVATE }}
+          private-key: ${{ secrets.PRIVATE_KEY }}
+          helmfile-src: "src/helm"
+          repositories: "meet,secrets"