8774: feat: Honor `.cargo/config.toml` r=matklad a=Veykril
Implements `cargo/.config` build target and cfg access by using unstable cargo options:
- `cargo config get` to read the target triple out of the config to pass to `cargo metadata` --filter-platform
- `cargo rustc --print` to read out the `rustc_cfgs`, this causes us to honor `rustflags` and the like.
If those commands fail, due to not having a nightly toolchain present for example, they will fall back to invoking rustc directly as we currently do.
I personally think it should be fine to use these unstable options as they are unlikely to change(even if they did it shouldn't be a problem due to the fallback) and don't burden the user if they do not have a nightly toolchain at hand since we fall back to the previous behaviour.
cc #8741Closes#6604, Closes#5904, Closes#8430, Closes#8480
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
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