Set NODE_ENV environment variable
Set NODE_ENV environment variable

Set the `NODE_ENV` environment variable, which is relied on by
Spotlight:
https://github.com/alphagov/spotlight/blob/74d1709210c2655fb5b43a57dea52715786c4741/app/server.js#L13

We can set this to the same value as `$govuk_env`, which will be
`production` in Preview, Staging and Production, and `development` on
the dev VM.

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