Roll buildroot to 4dfab138e7dcc4d174ad0032c102d0f9055ba904.

[rmacnak-google]

Download the right toolchain on linux-arm64.

Don't attempt to download non-existent Android tools for linux-arm64.

Allows building for desktop on a linux-arm64 host.

Bug: https://github.com/flutter/flutter/issues/103386