test(repo): Add a GITHUB TOKEN to the secret manager to avoid rate limits

### Determine this is the right repository

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### Summary of the feature request

The ad hoc package selection process during system tests performs a check against the GitHub (GH) API for a specific label. GH limits unauthenticated checks to 60 per hour per IP address.

Right now we are not hitting that limit, but we do not have control over any other kokoro processes that share our IP address, so we should provide a means for kokoro to authenticate and get nearly unlimited API calls.

For some context regarding where this call occurs: see https://github.com/googleapis/google-cloud-python/pull/17931#discussion_r3711547080

We will need to add the key to SECRET MANAGER in the Google Cloud Console.

### Desired code experience

### Expected results

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### API client name and version

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### Use case

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### Additional context

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Start with the ad hoc package selection process in system tests and review the linked discussion in google-cloud-python#17931 to locate the GitHub API label check. Then inspect how Kokoro accesses Google Cloud Secret Manager. Done means Kokoro can authenticate those checks with a stored GitHub token and the token is added to Secret Manager.

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