fix: install cross-env for server npm script

[ojeytonwilliams]

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I don't think this was ever reported as an issue, but it appeared on the forum https://www.freecodecamp.org/forum/t/cross-env-not-found/308753/2 and @RandellDawson noticed it.

The problem is simply that npm run, unlike require, does not search the parent directories. This affects docker, because it calls npm run develop from the api-server directory.

[gitpod-io]