Use zbindeps (moved to nightly)

[rukai]

This PR moves the project to nightly rust to make use of the unstable bindeps cargo feature.
This allows us to cleanly pull the firmware into the flasher tool without having to reinvoke cargo from within a build.rs.
The previous implementation was fine and I wouldnt recommend swapping to nightly just for this, but I think this feature is really cool and I would like to use this project as a testbed for the feature so I can contribute towards seeing it stabilized.

That said there are some really nice wins here:

• simplified build logic, cargo handles it all for me now.
• all the benefits of keeping the project in one workspace: cargo fmt, cargo clippy etc run on all the packages at once. Does not seem to be possible. Maybe one day... Instead I have a workspace including the flasher but excluding the firmware.
• rust-toolchains.toml was not being triggered by the inner cargo call resulting in build failures due to missing thumbv6m-none-eabi toolchain. Using bindeps fixes this issue.
• I can remove the manual workarounds from cargo-dist needed because the root of the repo wasnt the root of the crate. I could have just done this anyway by adding a workspace consisting only of the flasher, which I have now done.