💚(ci) improve submodule usage

- remove deplucate declaration
- simplify helmfile
- use symlink

.gitmodules

@@ -1,8 +1,3 @@
-[submodule ".github/workflows/secrets"]
-	path = .github/workflows/secrets
-	url = https://github.com/numerique-gouv/secrets.git
-	branch = main
-[submodule "src/helm/secrets"]
-	path = src/helm/secrets
-	url = https://github.com/numerique-gouv/secrets.git
-	branch = main
+[submodule "secrets"]
+	path = secrets
+	url = ../secrets