Fix cargo integration-test for Nix flake

[willbush]

The rustflags set in the flake.nix is stomping on helix/.cargo/config.toml:

rustflags = ["--cfg", "tokio_unstable", "-C", "target-feature=-crt-static"]

Here is the error I was getting:

$ cargo integration-test
   Compiling helix-event v24.7.0 (/home/will/code/helix/helix-event)
   Compiling helix-lsp-types v0.95.1 (/home/will/code/helix/helix-lsp-types)
   Compiling toml v0.8.19
   Compiling gix-url v0.27.5
error[E0412]: cannot find type `Id` in module `tokio::runtime`
   --> helix-event/src/runtime.rs:45:64
    |
45  |         parking_lot::RwLock<hashbrown::HashMap<tokio::runtime::Id, &'static T, ahash::RandomState>>,
    |                                                                ^^ not found in `tokio::runtime`
    |
note: found an item that was configured out
   --> /home/will/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.40.0/src/runtime/mod.rs:371:21
    |
371 |         pub use id::Id;
    |                     ^^

error[E0599]: no method named `id` found for struct `Handle` in the current scope
  --> helix-event/src/runtime.rs:67:52
   |
67 |         let id = tokio::runtime::Handle::current().id();
   |                                                    ^^ method not found in `Handle`

   Compiling gix-submodule v0.14.0
Some errors have detailed explanations: E0412, E0599.
For more information about an error, try `rustc --explain E0412`.
error: could not compile `helix-event` (lib) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...

Can temporarily fix this by running:

RUSTFLAGS="--cfg tokio_unstable" cargo integration-test

This fixes the error because https://docs.rs/tokio/1.40.0/tokio/runtime/struct.Id.html "Available on tokio_unstable and crate feature rt only."

So this PR moves the setting of flags for Linux under .cargo/config.toml.

I did some digging on the history of RUSTFLAGS in the nix files:

• f1539cc
• d5db892
• df0d58e

[willbush]

Note: seems to me nix isn't used in the github action when releasing so I would expect some change for GNU/Linux binary result compared to what flake.nix is doing (correct me if I'm wrong).

[willbush]

should I have used [target.'cfg(target_os = "linux")']? due to different targets in GH action release.yaml? Perhaps also the linker should be explicit..

[willbush]

Think the way I was trying to fix this was incorrect. Just looked at latest master and this fixed it