feat(cargo-codspeed): use cargo from cli

Local rustflags have be moved to cargo config.
Before this, the .vscode/settings.json was setting a `RUSTFLAGS` value
that was not shared by the cargo cli version, causing unneccessary
rebuilds between `cargo build` and an editor save.

Author: GuillaumeLagrange

.cargo/config.toml

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

.vscode/settings.json

@@ -1,7 +1,4 @@
 {
-  "rust-analyzer.cargo.extraEnv": {
-    "RUSTFLAGS": "--cfg codspeed"
-  },
   "rust-analyzer.cargo.features": "all",
   "editor.formatOnSave": true,
   "rust-analyzer.checkOnSave": true,