Rollup merge of #89930 - cuviper:avoid-clone3, r=joshtriplett

Only use `clone3` when needed for pidfd

In #89522 we learned that `clone3` is interacting poorly with Gentoo's
`sandbox` tool. We only need that for the unstable pidfd extensions, so
otherwise avoid that and use a normal `fork`.

This is a re-application of beta #89924, now that we're aware that we need
more than just a temporary release fix. I also reverted 12fbabd27f700, as
that was just fallout from using `clone3` instead of `fork`.

r? `@Mark-Simulacrum`
cc `@joshtriplett`

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