New review comment by ahesford on void-packages repository

https://github.com/void-linux/void-packages/pull/46263#discussion_r1462351631

Comment:
```suggestion
else
    : "${XBPS_MASTERDIR:=$XBPS_DISTDIR/masterdir-$XBPS_MACHINE}"
    if [ ! -d "${XBPS_MASTERDIR}" && -d "${XBPS_DISTDIR}/masterdir" ]; then
        XBPS_MASTERDIR="${XBPS_DISTDIR}/masterdir"
    fi
```
This avoids the need to break builds with existing `masterdir`.