Make the BES result clickable (#2623)

We already upload the BEP to the GCP Build Event Service (BES) API. This
change makes the resulting Build Test eXplorer (BTX) page clickable.

(dupe of #2622)

Example BES result:
https://btx.cloud.google.com/invocations/26a14903-0d90-4e77-9d3c-de4e4b264330/targets

From
https://buildkite.com/bazel/rules-android/builds/4498/canvas?sid=019e2cdc-d098-447b-ad14-636d484f5788&tab=output