tests: sandboxing

The slowness of bazel-zig-cc comes from setup and teardown of sandbox
directories[1]. Turning on `experimental_reuse_sandbox_directories`
speeds it up significantly.

Depending on how the tests with this go in the next few days, I will add
this recommendation to README.

This has been marked as safe to use and no longer experimental as of
Bazel master[2].

[1]: https://git.sr.ht/~motiejus/test-zigcc
[2]: https://github.com/bazelbuild/bazel/pull/16490

.bazelrc

@@ -4,5 +4,6 @@ test --test_output=errors
 build --verbose_failures
 build --worker_sandboxing
 
+build --experimental_reuse_sandbox_directories
 build --incompatible_enable_cc_toolchain_resolution
 build --action_env BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1