[hakari] Read rust-toolchain.toml

[P1n3appl3]

I was setting up hakari for zed recently, and realized when I was filling out the config that the [platforms] section of hakari.toml is (at least in our case) the exact same as the targets list in rust-toolchain.toml.

Would it make sense for hakari to use that list by default if you don't specify platforms in its own config? If so I'd be happy to make a PR doing that since it'd save me the effort of keeping them up to date by hand, I'm just not sure whether it's desirable in general.