ci: define top-level permissions for request-reviews and update-flake

[awwpotato]

cc @MattSturgeon @trueNAHO @danth

it seems unlikely this will fix our issues but I think it's worth a try.

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[danth]

This might be the real cause: #1053 (comment)

[trueNAHO]

This might be the real cause: #1053 (comment)

I tried this in #1848.