Follow-up to:
- https://github.com/sigp/lighthouse/pull/7764
The `heaptrack` feature added in my previous PR was ineffective, because the jemalloc feature was turned on by the Linux target-specific dependency.
This PR tweaks the features such that:
- The jemalloc feature is just used to control whether jemalloc is compiled in. It is enabled on Linux by the target-specific dependency (see `lighthouse/Cargo.toml`), and completely disabled on Windows.
- If the `sysmalloc` feature is set on Linux then it overrides jemalloc when selecting an allocator, _even if_ the jemalloc feature is enabled (and the jemalloc dep was compiled).