Update required for FP4 install script to support MacOS environment

Issue as reported by Gaël

Regarding the Fairphone installation procedure, I noticed that the script we ship doesn't handle the MacOS case, which obviously makes it fail on MacOS. Is there any reason for this? This is a 4 line fix in the script source code (+ embedding the Fastboot binary in the archive) (see https://mastodon.social/@gael/110946096040492650 for context)