gh-35735: gap: use CALL_WITH_STREAM to redirect output to string
    
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### 📚 Description

This avoids use of deeply internal GAP APIs which may change at any
time. In particular `TypOutputFile` is likely to change again and again.

In contrast, `CALL_WITH_STREAM` is on a much higher level (and even
reachable from the GAP language itself). We have no plans to change it.

Together with PR #35702 this resolves #35361.

### 📝 Checklist

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### ⌛ Dependencies

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URL: #35735
Reported by: Max Horn
Reviewer(s): Frédéric Chapoton