Store marker_rustc_driver with the toolchain, that it belongs to

[xFrednet]

Marker's rustc driver is bound to a specific nightly toolchain version. When running the driver, we need to be certain, that the correct toolchain is used.

The best solution I see right now, would be to use rustup as a proxy like it's used for most tools. This would require:

• Creating the rustup file called marker_rustc_driver
• Installing the marker_rustc_driver in the correct toolchain folder
• Ensure the invocation uses the right path (rustup +<toolchain> which marker_rustc_driver should work just fine)

A project with a rust-toolchain file might interfere with this. This needs to be tested.

[xFrednet]

It turns out, that rustup, has a hard-coded list of valid proxy names. This means that we can't use it directly like I planned. However, rustup +<toolchain> which marker_rustc_driver works just fine, as that command doesn't check the tool name :)

https://github.com/rust-lang/rustup/blob/843459f893091f89b2dd5c7a977da1b3eb28df1b/src/lib.rs#L42-L61