fix: use toolchain file while building dependencies

[LtdSauce]

This commit make the cargo build command that builds the dependencies actually adhere to the toolchain file if it was present in the recipe.

Before this it was ignored and thus the next call to cargo build would rebuild all dependencies again, but then with what is actually contained in the toolchain file. This was because the environment variable 'RUSTUP_TOOLCHAIN' was already set and so the toolchain file was not considered, see here for details:
https://rust-lang.github.io/rustup/overrides.html

This should fix #271 (and also solve the missing piece for #231)

[LtdSauce]

Hi! I have not found tests, that actually check the cargo invokation and thus did not add any tests for this.
I have tested it locally though by executing it with this fix and saw that the toolchain file was not recognized before it, but after the patch it was.

This could be reproduced using any recipe that contains a toolchain, that uses a different channel/version than what is installed by default. I used the following for example:
recipe.json

[LukeMathWalker]

Coming back to this way too late, nonetheless: I don't think this is the right approach.
As far as I can see, the default Rust images don't set RUSTUP_TOOLCHAIN. If the user is setting that environment variable, then cargo-chef shouldn't undermine the user intention unconditionally.

Based on my current understanding, we're already doing the right thing here: we restore the toolchain file and then let rustup figure out, based on its hierarchy, whether it should be honored or not.

I'll close this, but feel free to re-open if my reasoning is incorrect.