Increase the number of 'X' characters in calls to `mktemp`, for portability.

On OpenBSD, using only five 'X' characters results in the following error message:

mktemp: insufficient number of Xs in template `tmp_bazel_zip_files_XXXXX'

On OpenBSD, the minimum number of 'X' characters is six:
https://man.openbsd.org/mktemp

PiperOrigin-RevId: 315462488

src/zip_files.sh

diff --git a/src/zip_files.sh b/src/zip_files.sh
index 73926e5..1422a6c 100755
--- a/src/zip_files.sh
+++ b/src/zip_files.sh
@@ -31,7 +31,7 @@
 
 initial_pwd="$(pwd)"
 
-tmp_dir=$(mktemp -d -t 'tmp_bazel_zip_files_XXXXX')
+tmp_dir=$(mktemp -d -t 'tmp_bazel_zip_files_XXXXXX')
 trap "rm -fr $tmp_dir" EXIT
 tmp_zip="$tmp_dir/archive.zip"