Fix environment variable interpolation syntax

- Change {BORG_REPO} to ${BORG_REPO} for correct Borgmatic interpolation
- The $ prefix is required for environment variable substitution
- Fixes "Invalid placeholder" error when running borgmatic

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

config.yaml

@@ -5,7 +5,7 @@
 
 # Repository Borg - utilise la variable d'environnement
 repositories:
-    - path: "{BORG_REPO}"
+    - path: "${BORG_REPO}"
       label: serveur
 
 # Chemins sources à sauvegarder