Make "publish binaries" pipeline more robust

Example failure:

https://buildkite.com/bazel-trusted/publish-bazel-binaries/builds/8525#7ce8bb29-4615-470e-a9aa-d20ede99e03d

This was an intermittent failure (all other builds, including a retry, succeeded) that went unnoticed until a user reported problems with Bazelisk: https://github.com/bazelbuild/continuous-integration/issues/1074

Two action items:
1. Investigate this particular flake
2. Upload as many binaries as possible, even if some tasks failed. Currently the upload step only runs if all tasks succeed.

Start with the failed Buildkite run for the publish-bazel-binaries pipeline and compare it with the successful retry and the related Bazelisk report. Trace the pipeline's task and upload steps; done means identifying the intermittent failure and ensuring available binaries are uploaded when individual tasks fail.

Written by the indexing model from the issue text.