Draft: Use /usr/X instead of /X

Roland Clobusrequested to merge
rclobus/live-build:rclobus/usr-merge into master

Use /usr/bin, /usr/lib and /usr/sbin in the scripts, instead of the variant without /usr

Tested by booting a GNOME image for bullseye, bookworm, trixie and sid. Each has symlinks in place, so there is no need for transitional comments.

Closes #1064408

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