Handle the case when OUT_DIR is not set.

Bug: 137977523
Test: manual
Change-Id: I6a4bcc5099223f28bce72bf10004c01327598fc7
diff --git a/build_kzip.bash b/build_kzip.bash
index 62ea7a2..5364e7f 100755
--- a/build_kzip.bash
+++ b/build_kzip.bash
@@ -22,4 +22,4 @@
 # Pack
 # TODO(asmundak): this should be done by soong.
 declare -r allkzip=all.kzip
-"$OUT_DIR/soong/host/linux-x86/bin/merge_zips" "$DIST_DIR/$allkzip" @<(find $OUT_DIR -name '*.kzip')
+"${OUT_DIR:-out}/soong/host/linux-x86/bin/merge_zips" "$DIST_DIR/$allkzip" @<(find $OUT_DIR -name '*.kzip')