Allow writing test results to disk instead of uploading

[marcosgriselli]

Allows projects using Bazel test result caching to avoid invalidating cache when passing all the Buildkite environment flags.

Happy to submit a PR if this feature is welcome.

[KatieWright26]

Hi @marcosgriselli, thanks for raising this. At the moment we aren't working on the swift collector, however I'd like to leave this issue open for others to 👍 so that we can gauge the desire for this feature in the future.