ci: overwrite runner event file so reviewdog detects fork PRs

[openroad-ci]

Summary

clang-tidy-bazel-post posted no review comments on fork PRs — reviewdog exited with reviewdog: this is not PullRequest build. on every run (#10550, #10553, #10226, #10565).

Root cause (confirmed)

GITHUB_EVENT_PATH and GITHUB_EVENT_NAME are runner-protected default env vars. The step-level env: override we used to point reviewdog at a synthesized pull_request payload is silently dropped — the runner re-injects the real workflow_run event file, whose payload has no usable pull_request (its pull_requests[] is empty for fork PRs). reviewdog's cienv parses that file, sees PullRequest == 0, and bails before posting. Our synthesized side file was correct but never read.

Proof, from the debug-gated diagnostic step on public run 26681282929 attempt 2 (PR #10565) — the step set both overrides, yet env inside it reported:

GITHUB_EVENT_NAME=workflow_run                                         # wanted pull_request
GITHUB_EVENT_PATH=/home/runner/work/_temp/_github_workflow/event.json  # wanted our synth file

and that runner file carried "pull_requests": [].

Fix

Stop overriding the env pointer (a no-op). Instead overwrite the runner's event file in place with the synthesized pull_request payload. reviewdog's cienv only parses the file at GITHUB_EVENT_PATH (it does not check GITHUB_EVENT_NAME), so the overwrite alone makes detection succeed. Owner/repo/head-sha all come from the payload we write, so the now-dead GITHUB_* overrides on the reviewdog step are removed. Nothing after the synth step depends on the original workflow_run event, so overwriting it is safe.

The debug-gated diagnostic step (runner.debug == '1') is kept and updated to verify the event file now holds the synthesized payload.

Test plan

• After landing through the mirror chain, confirm reviewdog posts clang-tidy findings as inline review comments on a fork PR (e.g. re-use the test: trigger fresh clang-tidy-bazel-post for reviewdog diag #10565 test branch).
• Confirm normal runs are unaffected (diag step skipped when not a debug re-run).

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.