More CI (#3)

- ci only: use a separate zig cache dir for each run
- use `tools/bazel` everywhere.
- remove arm64 tests. They don't give enough value to be worth the
  brittle environment.
- remove windows execution, except for launcher. Ditto. I will probably
  bring them back.

.bazelrc

@@ -7,3 +7,5 @@ build --worker_sandboxing
 build --experimental_reuse_sandbox_directories
 build --incompatible_enable_cc_toolchain_resolution
 build --action_env BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
+
+try-import %workspace%/.custom.ci.bazelrc