hack/ci: exit instead of return on missing github credentials

[ROKUMATE]

pr-should-include-tests is executed, not sourced, so the return 1 used when GITHUB_TOKEN or GITHUB_REPOSITORY is unset does not stop the script. bash prints return: can only return from a function or sourced script and execution falls through to the curl call with empty credentials, ultimately failing with the misleading PR does not include tests message instead of the intended cannot query github error.

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Does this PR introduce a user-facing change?

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[Honny1]

LGTM

PTAL @ashley-cui @timcoding1988

[Luap99]

why not just turn this into exit 1?

This gave a reasonable error message, with the change now we get the full no tests message if GITHUB_TOKEN/GITHUB_REPOSITORY is missing from the env which will be confusing.