WORKSPACE: bump bazel to 6.4.0

Change-Id: Id6ca8317c1eaeb55dc8f272fa51f8879c4c02320
Reviewed-on: https://review.monogon.dev/c/monogon/+/2261
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
diff --git a/.bazelversion b/.bazelversion
index 8a30e8f..19b860c 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-5.4.0
+6.4.0