feat: external and persisted Zig cache

[aherrmann]

Configure an external, globally shared, and persisted cache directory for the Zig compiler. This directly follows the example of uber/hermetic_cc_toolchain.

Closes #87

This is required for Zig 0.12 support as Zig 0.12 produces dangling symlinks in its cache directory, making the current approach of treating the cache directory as a build output infeasible, see bazelbuild/bazel#15454 (comment).

Time saved due to persisted Zig cache

$ rm -rf /tmp/zig-cache; \
  bazel clean; \
  bazel build //configure-target:binary_x86_64-windows \
    --noremote_accept_cached; \
  bazel clean; \
  bazel build //configure-target:binary_x86_64-windows \
    --noremote_accept_cached
...
INFO: Elapsed time: 6.784s, Critical Path: 3.83s
...
INFO: Elapsed time: 4.047s, Critical Path: 1.22s

• Configure Zig cache directory from environment
• Track Zig cache directory in toolchain info
• Configure Zig cache command line options
• Configure sandbox mount path for Zig cache
• update generated files
• Move env_zig_cache_prefix into zig_cache.bzl
• Unit test for the Zig cache configuration logic
• Test match toolchain and build cache configuration
• Integration test that Zig cache can be configured
• Document Zig cache configuration