Use a concrete bazel version. (#387)

Bazel does not support semantic versioning properly, so 6.x only really
works with bazelisk, which downloads binaries from the Internet.

Instead, use a concrete version to make sure bazel can correctly
deal with it (also bazelisk if someone wants to use that).

.bazelversion

@@ -1 +1 @@
-6.x
+6.5.0