common/xbps-src/shutils/update_check: add ability to disable update-check

useful for cases where update checking does not make sense or is
impossible

common/xbps-src/shutils/update_check.sh

@@ -13,6 +13,9 @@ update_check() {
         if [ "$XBPS_UPDATE_CHECK_VERBOSE" ]; then
             echo "using $XBPS_TARGET_PKG/update overrides" 1>&2
         fi
+        if [ -n "$disabled" ]; then
+            echo "update-check DISABLED for $original_pkgname: $disabled" 1>&2
+        fi
     elif [ -z "$distfiles" ]; then
         if [ "$XBPS_UPDATE_CHECK_VERBOSE" ]; then
             echo "NO DISTFILES found for $original_pkgname" 1>&2