Add 'ob deploy build' command

[tomsmalley]

As discussed in https://github.com/obsidiansystems/obelisk/issues/368

I have:

• [x] Based work on latest develop branch
• [x] Followed the contribution guide
• [x] Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• [ ] Run the test suite: $(nix-build -A selftest --no-out-link)
• [x] Updated the changelog
• [ ] (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[tomsmalley]

There's also potential to add the checks done by ob deploy init:

https://github.com/obsidiansystems/obelisk/blob/d89b1a6b696903df0dd74f93bdc9c847ff99b80b/lib/command/src/Obelisk/Command/Deploy.hs#L77-L82

as Ryan mentioned, but I didn't think it made much sense for ob deploy build. I can add ob deploy prepare which would run these checks on top of the build.