ci: add rustfmt check

Signed-off-by: oppiliappan <[e-mail]>

.tangled/workflows/fmt.yml

···
+when:
+  - event: ["push", "pull_request"]
+    branch: ["main"]
+
+dependencies:
+  nixpkgs:
+    - cargo
+    - rustc
+    - rustfmt
+
+steps:
+  - name: "cargo fmt"
+    command: |
+       cargo fmt --all --check