Add box64 wrapper to enable running Dropbox source builds on ARM64 Ubuntu.

[jewelpit]

1. This is purely for making it a bit easier to run on SBC machines and Apple Silicon VMs. This is not a serious build.
2. The tray icon doesn't work and we're not going to produce an official package. Source builds only.
3. Due to multiarch package difficulties, ARM64 Fedora is 100% unsupported, unlike ARM64 Ubuntu, which is 99% unsupported.
4. A native ARM build is unlikely any time soon.

Within the above constraints, have fun!