Draft: testing/bazel6: upgrade to 6.4.0

https://github.com/bazelbuild/bazel/releases/tag/6.4.0

Continuing work from !49467 (closed) after learning more about how to manage Alpine build environments. Successfully patched issues relating to removed lfs64 fallback under musl, and updates for GCC 13.

I need help with Bazel here. It seems to be pulling and building external code rather than using the locally available version, e.g. there are errors building a remote version of src/tools/singlejar/port.h when this file is already patched locally. How can I make it use the local version?