Fix nightly upgrade workflow GitHub API rate limiting

[svarlamov]

Avoid GitHub API rate limit failures in nightly upgrade workflow

Summary

The nightly upgrade validation workflow was intermittently failing with HTTP Error 403: rate limit exceeded when calling the GitHub Releases API unauthenticated (shared runner IPs can quickly exhaust the unauthenticated limit).

This PR:

• Passes secrets.GITHUB_TOKEN into the “Select random older versions” steps (bash + Windows pwsh)
• Uses Authorization: Bearer $GITHUB_TOKEN (when present) and adds an explicit User-Agent header for the GitHub API request

Review & Testing Checklist for Human

• Confirm GITHUB_TOKEN is available for the scheduled workflow in this repo/org (permissions/policy) and that this doesn’t require any additional workflow permissions.
• Sanity-check the Windows step: the Python snippet is embedded in a PowerShell here-string; verify the updated block is syntactically correct and still executes on windows-latest.
• Optionally re-run the workflow (or a workflow_dispatch) and confirm the “Select random older versions” steps no longer fail with 403 rate limits.

Notes

• Link to Devin run: https://app.devin.ai/sessions/542cbe6f96cc42d6b9e173485666d7b0
• Requested by: @svarlamov

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.