Add rustflags in cargo config (#543)

* add rustflag in cargo config

* add documentation of where the rustflags are set.

* #543 (comment)

* #543 (comment)

.cargo/config.toml

@@ -0,0 +1,2 @@
+[build]
+rustflags = ["--cfg", "tokio_unstable"]

clients/README.md

@@ -219,9 +219,13 @@ due to the converted destination account not having trustlines set up for the re
 The destination account is derived automatically from the account that called the extrinsic on-chain.
 
 ### Debugging with `tokio-console`
-The vault is `tokio-console` ready, with the feature **_`allow-debugger`_**. _Remember to [set the rustflags](https://github.com/tokio-rs/console?tab=readme-ov-file#instrumenting-your-program)!_
+The vault is `tokio-console` ready, with the feature **_`allow-debugger`_**. 
+
+> **Note**: [Rustflags are required](https://github.com/tokio-rs/console?tab=readme-ov-file#instrumenting-your-program) to make `tokio-console` work.
+It has been defined directly in [.cargo/config.toml](../.cargo/config.toml). 
+
 ```
-RUSTFLAGS="--cfg tokio_unstable" cargo run --bin vault --features allow-debugger
+cargo run --bin vault --features allow-debugger
 ```
 [Install tokio-console](https://github.com/tokio-rs/console?tab=readme-ov-file#running-the-console)
 and connect to the vault.