Add a config.toml symlink (#3503)

This also uses `cargo xtask` in the action, to test this functionality

.cargo/config.toml

@@ -0,0 +1 @@
+../config.toml

.github/workflows/checks.yml

@@ -87,8 +87,8 @@ jobs:
 
       - uses: actions-rs/cargo@v1.0.3
         with:
-          command: run
+          command: xtask
-          args: --package xtask -- codegen
+          args: codegen
 
       - name: check for changes
         run: |