Switch to the Haswell CPU microarchitecture
Switch to the Haswell CPU microarchitecture

This changes the `rustc` flag `target-cpu` to use Haswell, rather than
the CPU of the host machine (the machine building Serenity and other
crates, like the examples).

This change was brought on by errors from the CI job runners aborting
due to "Illegal instruction"s. To speed things up, we employ a cache in
CI to avoid recompiling dependencies as much as possible, but this
causes problems when `target-cpu` is set to `native`. This is because
the job runners might have different CPUs than the job that build and
saved the cache, causing the above "Illegal instruction" error as the
runners' CPUs may not have the necessary instructions as the build job.

As a compromise, we will now use Haswell, which has the necessary
SIMD instructions for building `simdjson`.

acdenisSK/serenity,Flat/serenity