ci/fdno: Only run full tests on a limited subset of machines

Though not many people run full test runs, it occupies 2/7 a630 slots
for nearly 2 hours. If more than one person does this at a time, it can
be an effective DoS and make merges time out.

Limit full runs to a subset of the runners, such that at least some of
them will always be available for us.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21737>
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