Add task `kythe_ubuntu2004` to `postsubmit.yml`
Add task `kythe_ubuntu2004` to `postsubmit.yml`

Add task`kythe_ubuntu2004` to generate indexing files with Kythe and upload to GCS automatically.

Must be merged after https://github.com/bazelbuild/continuous-integration/commit/7bb698c41cd6159475154e74c5fab3a38b6106b5 landed.

Closes #12011.

PiperOrigin-RevId: 329437289

ButterflyNetwork/bazel,twitter-forks/bazel,davidzchen/bazel,bazelbuild/bazel,safarmer/bazel,perezd/bazel,cushon/bazel,katre/bazel,werkt/bazel,werkt/bazel,katre/bazel,twitter-forks/bazel,werkt/bazel,werkt/bazel,katre/bazel,davidzchen/bazel,twitter-forks/bazel,meteorcloudy/bazel,davidzchen/bazel,bazelbuild/bazel,werkt/bazel,davidzchen/bazel,safarmer/bazel,ButterflyNetwork/bazel,bazelbuild/bazel,davidzchen/bazel,bazelbuild/bazel,perezd/bazel,meteorcloudy/bazel,cushon/bazel,katre/bazel,ButterflyNetwork/bazel,werkt/bazel,bazelbuild/bazel,perezd/bazel,katre/bazel,safarmer/bazel,twitter-forks/bazel,cushon/bazel,safarmer/bazel,meteorcloudy/bazel,twitter-forks/bazel,perezd/bazel,ButterflyNetwork/bazel,meteorcloudy/bazel,cushon/bazel,meteorcloudy/bazel,twitter-forks/bazel,cushon/bazel,perezd/bazel,perezd/bazel,perezd/bazel,ButterflyNetwork/bazel,meteorcloudy/bazel,davidzchen/bazel,cushon/bazel,katre/bazel,safarmer/bazel,bazelbuild/bazel,ButterflyNetwork/bazel,twitter-forks/bazel,davidzchen/bazel,meteorcloudy/bazel,safarmer/bazel