Magic Leap nightly release in CI

[asajeffrey]

Steps to do nightly releases of the Magic Leap build:

• Put the ML temp signing key on S3, so it can be provisioned by salt. (servo/saltfs#918)
• Add a CI nightly job to build and release the Magic Leap app. (#22180)
• Hook the nightly job into homu. (servo/saltfs#920)

Building the app only takes a few seconds, so we can run that on every merge, which will help catch link errors.

[asajeffrey]

Depends on #22078.

[highfive]

cc @aneeshusa

[asajeffrey]

All the steps are there, will there be nightlies in S3 tomorrow? We shall see...

[asajeffrey]

And there are! http://servo-builds.s3.amazonaws.com/nightly/magicleap/servo-latest.mpk