cmd/storj-sim: expose GATEWAY_X_API_KEY

[wthorp]

What:
Expose GATEWAY_X_API_KEY in storj-sim network env

Why:
gateway setup or any similar use of libuplink has a setup phase which requires an API Key
there's already one created, it just wasn't easy to get at

Please describe the tests:

• Test 1: none

Please describe the performance impact:
none

Code Review Checklist (to be filled out by reviewer)

• Does the PR describe what changes are being made?
• Does the PR describe why the changes are being made?
• Does the code follow our style guide?
• Does the code follow our testing guide?
• Is the PR appropriately sized? (If it could be broken into smaller PRs it should be)
• Does the new code have enough tests? (every PR should have tests or justification otherwise. Bug-fix PRs especially)
• Does the new code have enough documentation that answers "how do I use it?" and "what does it do?"? (both source documentation and higher level, diagrams?)
• Does any documentation need updating?
• Do the database access patterns make sense?

[ethanadams]

Works for me.

[mobyvb]

It works :)