Upload nightly binary installer snapshots.

[brson]

After #9876, begin uploading nightly installer snapshots. They can probably all just have the same name, overwriting each other every night.

[brson]

Nominating.

[catamorphism]

1.0, high

[alexcrichton]

@brson has also done some amazing work on this front, we have nightlies for linux/mac/windows, hurray!