CI: use global config toml (#3384)

It looks like we should keep RUSTFLAGS consistent in CI if possible;
some commands augmented RUSTFLAGS with "-D warnings" which overrode
`.cargo/config.toml`, causing unnecessary rebuilds even for non-bundling
runs. Tl;dr: for the last few days our average CI time spiked
significantly.
There are several solutions:
- We can place `-D warnings` in our `.cargo/config.toml`. That's not a
good solution, because then you wouldn't ever be able to build Zed with
warnings locally. A true PITA!
- We can place another config.toml somewhere in the search path
(https://doc.rust-lang.org/cargo/reference/config.html#hierarchical-structure)
and rely on the merging of properties. That way we can avoid having `-D
warnings` on developer machines while being able to override CI
behaviour at will.

This PR implements the latter approach by creating the new config file
manually. Ideally we should have it a a separate file in repository
that's moved into $HOME/.cargo on each CI run. Maybe we should even
place it somewhere more local to the checked out Zed version, as placing
it in a global spot is kinda bad too - what if we start building
multiple cargo projects on our CI machines?

Release Notes:

- N/A