xbps-src: print_cross_targets properly without `XBPS_DISTDIR` set

because it is called in `usage()`, before `XBPS_DISTDIR` is set by `xbps-src`,
it would only work if `XBPS_DISTDIR` is set by the user.

xbps-src

@@ -9,6 +9,13 @@ print_cross_targets() {
 }
 
 usage() {
+    # copied from main logic because this is run before XBPS_DISTDIR is set normally
+    local _distdir="$(readlink -f "${0%/*}")"
+    if [ "${_distdir}" = "." ]; then
+        readonly XBPS_DISTDIR="$(pwd -P)"
+    else
+        readonly XBPS_DISTDIR="${_distdir}"
+    fi
     cat << _EOF
 $PROGNAME: [options] <target> [arguments]