Use the Primer GitHub App for auth instead of the GPR_AUTH_TOKEN_SHARED

[camertron]

This PR replaces usage of the GPR_AUTH_TOKEN_SHARED PAT with a token obtained through the Primer GitHub App in several CI scenarios.

Fixes https://github.com/github/primer/issues/2540

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Latest commit: 5cdecd9

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