New review comment by sgn on void-packages repository

https://github.com/void-linux/void-packages/pull/27582#discussion_r560926885

Comment:
Bikeshedding, I actually prefer something like:
```sh
local _postfix=
case "$XBPS_TARGET_MACHINE" in
     aarch64*) _postfix="/aarch64" ;;
     *-musl) _postfix="/musl" ;;
esac
echo "repository=https://alpha.de.repo.voidlinux.org/current$_postfix" \
    >${DESTDIR}/usr/share/xbps.d/00-repository-main.conf
```