GH Actions/reusable-build-binary.yml: cache download of box PHAR

[jrfnl]

From: #185 (review)

@theofidry suggested:

side-note: might be worth caching it.

You could check https://github.com/box-project/box/releases/latest for which the header location gives the tag (e.g. https://github.com/box-project/box/releases/tag/4.6.10) and use that version as the cache key