alpine-conf: setup-alpine typo breaks use of an answers file

A change to perform some minor clean up in the "setup-alpine" script introduced a typo on line 64 which interferes with the use of an answers file:

-- 8< --
esac
if [ -n "$USERANSERFILE" ] && [ -e "$USEANSWERFILE" ]; then
	. "$USEANSWERFILE"
fi
-- 8< --

The USERANSERFILE should be: USEANSWERFILE

Reference: https://gitlab.alpinelinux.org/alpine/alpine-conf/-/blob/master/setup-alpine.in#L64

This may be easily worked around by setting USERANSERFILE when invoking "setup-alpine":

USERANSERFILE=1 setup-alpine -f ./myanswerfile