pmb.sideload: Run apk incantation in a subshell
I have Nushell set up on my Librem 5, and it is not a POSIX-compatible shell. As such, the rc=$? variable assignment in this snippet fails. Running this in a sh subshell works around this problems and will likely fix it for other non-POSIX shells as well given that /bin/sh should at least be POSIX-compatible on any sensible system.