Use a custom postsubmit.yml for Bazel in downstream (#2165)

Addresses
https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/4431#0194b040-d16f-44ea-abd5-f11ed5e82f95

We can revert to the official one after upgrading Bazel to 8.1.0 which
would contain
https://github.com/bazelbuild/bazel/commit/59cb6fe990933eb764fd22ef3115c0d7f51baded
3 files changed