Get rid of the `test-miri` flag in `config.toml`

[oli-obk]

The test-miri flag changes whether -Zalways-encode-mir is passed to rustc, causing all metadata beyond stage 0 to include MIR of all functions, even private monomorphic ones. Instead, what we can do is to have miri tests first run cargo miri setup to generate that special libstd in CI in addition to the regular libstd. Then tests can be run with special libstd.

cc @RalfJung

[RalfJung]

Worse, it also passes --cfg miri to rustc, meaning we get the hashbrown without SIMD!

[RalfJung]

Oh also, ideally this could use or at least share some code with the build script Miri uses to do the same on its own CI.

EDIT: Or maybe not, that does some crazy RUSTFLAGS stuff that probably should not be done for the released artifact.

[RalfJung]

I submitted a PR for this at #63162.