`nix build --system <system>` is not documented in `nix build --help`

[doronbehar]

Problem

As stated in issue title.

Proposal

Document it in nix build --help.

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

To be slightly less accurate, I simply ran:

git grep -- '--system'

On the repo's latest (61d0758) master, and found --system mentioned only in the docs of nix-env.

Priorities

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