[Fix] Correctly set testcase build url and gn args for fuzzing on batch

[ViniciustCosta]

Fix missing testcase metadata when fuzz task is running on batch (or any non-local execution).

Context

Some build-related environment variables (build_url, build_key and gn_args_path) are set during the build setup on fuzz task main stage. These should be retrieved during postprocess to set the testcase's metadata, which is used to display the build url and the gn args config section on the clusterfuzz UI.

However, for non-local execution, these variables are not propagated through the uworker_output proto and are missing in postprocess, so any testcase created during fuzzing on batch is not displaying these values.

Changes

• Added the build_url, build_key and gn_args fields to the fuzz task output message.
• Added the logic to store these fields and use them to set the initial testcase metadata.

Tests

• Testing on development environment is ineffective as it is not currently running fuzz tasks on batch. I deployed to dev anyway and confirmed that, at least, the local execution of fuzz tasks is still working.

• Testing on prod (deployed this PR to internal)

  • Fuzzing hours stable after deploy: https://screenshot.googleplex.com/7JfregzBR8F6MRA.png
  • Logs for fuzz tasks executing on batch: https://cloudlogging.app.goo.gl/5xXpLjUdwzK6wTeR9 (main) and https://cloudlogging.app.goo.gl/nX832G1782asdMDXA (postprocess)
  • Logs for fuzz tasks executing on GCE VMs: https://cloudlogging.app.goo.gl/61nvxR1BwQrQnvXr8 (main) and https://cloudlogging.app.goo.gl/6yoQSvU8r1mUVxsN7 (postprocess)

• Evidence of testcases creates after the deploy containing the build url and GN config args:

  • GCE VMs (local exec): https://clusterfuzz.com/testcase-detail/6746169883426816
  • Batch: https://clusterfuzz.com/testcase-detail/6451774202249216
    • Build URL: https://screenshot.googleplex.com/45n7M75hEPoEsNZ.png
    • GN Configs: https://screenshot.googleplex.com/9r3SxRPRQRyC4tv.png

Bug: b/441128474
Related issue: #4914

[decoNR]

Nice improvement :)

[javanlacerda]

Could you provide any evidence this changes will not break anything in prod?

[ViniciustCosta]

Could you provide any evidence this changes will not break anything in prod?

I added to the PR description. PTAL