Magic Leap build in CI

[asajeffrey]

We're currently not running any tests for the --magicleap build in CI. Steps to do this:

• Put the ML SDK on S3, so it can be provisioned by salt.
• Install the SDK on the Mac builders. (servo/saltfs#908)
• Add a CI job to run the build. (#22117 and servo/saltfs#910)

AFAICT there's not really any way to run a smoke test, since we don't have ML hardware in CI, but at least this would keep the build alive.

[asajeffrey]

Should we also package the ML app and do a nightly release for it? Or hold off on that until we're happier with the UI?

[jdm]

I would be happy to start publishing nightly builds with #22076 merged and these checkboxes checked.

[jdm]

By which I mean, yes, #22079 would be necessary before we publish nightly builds.

[asajeffrey]

Ok. Sounds like #22076 is the next step.

[asajeffrey]

Argh, I meant #22079.

[asajeffrey]

All merged now, though we're not gating on the ML build.