chore/CI: place .cargo/config.toml augmentations in ~/.cargo/config.toml

.github/workflows/ci.yml

@@ -23,6 +23,9 @@ jobs:
       - self-hosted
       - test
     steps:
+      - name: Set up default .cargo/config.toml
+	run: echo "[build]\nrustflags = [\"-D\", \"warnings\"]" > $HOME/.cargo/config.toml
+
       - name: Checkout repo
         uses: actions/checkout@v3
         with: