setup-alpine: add USERSSHKEY option to answerfile

Add a variable for user's ssh key, USERSSHKEY. We need this because the
key is space separated and we want avoid parse USEROPTS with eval to
handle quotes.

fixes https://gitlab.alpinelinux.org/alpine/alpine-conf/-/issues/10520
1 job for master in 23 seconds (queued for 1 second)