External interpreter GHCi tests are fragile on AArch64/Darwin in 9.0.2

For reasons that have evaded me, the GHCi tests (particularly those involving the debugger) are quite fragile on AArch64/Darwin in the ghc-9.0 branch (but strangely not in master or ghc-9.2). This was first noted in !6712 (merged).

I will mark them as fragile.

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