fix: arithmetic increment failing with set -e in nix-update workflow

.gitea/workflows/nix-update.yml

@@ -167,7 +167,7 @@ jobs:
             for pkg in $UPDATABLE_PACKAGES; do
               (run_update "$pkg" && echo "$pkg" >> /tmp/success.txt || true) &
 
-              ((JOB_COUNT++))
+              JOB_COUNT=$((JOB_COUNT + 1))
 
               # Wait if we hit max concurrent jobs
               if [ $JOB_COUNT -ge $MAX_JOBS ]; then