Mini-Oxygen shouldn't blow up when .env is missing from the local dir

[jplhomer]

Yeah people can do whatever they want with this, so we shouldn't blow up when .env is missing from the local dir.

> shopify hydrogen dev --entry ./server


🏁 Initial build: 479.869ms
🚥 MiniOxygen server started at http://localhost:3000

/Users/joshlarson/src/github.com/Shopify/h2/node_modules/yoga-layout-prebuilt/yoga-layout/build/Release/nbind.js:53
        throw ex;
        ^

[Error: ENOENT: no such file or directory, open '/Users/joshlarson/src/github.com/Shopify/h2/templates/hello-world/.env'] {
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: '/Users/joshlarson/src/github.com/Shopify/h2/templates/hello-world/.env'
}