build: add node GC argument to fly template

Fly deployments on the free tier have ~256mb of memory available. Users
with large transaction histories were encountering out of memory errors
when attempting to export their data. This commit adds a node argument
to (more or less) run the garbage collector at a smaller memory usage,
helping keep users on flyio within their available limit.

fly.template.toml

@@ -11,6 +11,7 @@ processes = []
 [experimental]
   allowed_public_ports = []
   auto_rollback = true
+  cmd = ["node", "--max-old-space-size=180", "app.js"]
 
 [[services]]
   http_checks = []