Support `miriflags` in `[target]` configuration

[nazar-pc]

Problem

I've been trying to use -Znext-solver=globally recently and due to how invasive it is, I ended up specifying it in the configuration file for both rustflags and rustdocflags. Unfortunately, miriflags (to mirror MIRIFLAGS environment variable) isn't supported at the moment.

Proposed Solution

Support miriflags under [target] configuration in the config file alongside existing rustflags and rustdocflags, such that it is possible to do this:

[target.'cfg(all())']
rustflags = ["-Znext-solver=globally"]
rustdocflags = ["-Znext-solver=globally"]
miriflags = ["-Znext-solver=globally"]

Notes

I have no idea why -Znext-solver=globally isn't inherited from rustflags for both, but that is a separate issue (for example, #17045).

[epage]

cargo miri is a first-party plugin managed in another repo. Unsure if they even could imitate rustflags with any degree of fidelity.

cc @RalfJung

[RalfJung]

Also see rust-lang/miri#2347. This would definitely need some help from cargo, not sure if the infrastructure for that exists or what it should look like.

That said, I am a bit surprised that rustflags doesn't apply. I was expecting it would. The RUSTFLAGS environment variable does apply with cargo miri.