Simplify `jemalloc` setup (without perf regression)
Reland https://github.com/rust-lang/rust/pull/146627 after fixing [the performance regression](https://github.com/rust-lang/rust/pull/148851#issuecomment-3525797560) that caused it to be reverted in https://github.com/rust-lang/rust/pull/148896.
This avoids
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