Make self-hosted buck2 optimised

[ndmitchell]

From https://github.com/facebook/buck2/blob/f014edfff62a78dc2868c580edb8950f456a65ec/docs/bootstrapping.md:

Note that the resulting binary will be compiled without optimisations or jemalloc, so we recommend using the Cargo-produced binary in further development.

We should make the self-compiled Buck2 optimised in these ways.

[ndmitchell]

Might be a good thing to do after we have modifiers, to make it easy to express.