CI: make rustfmt work in nix

.drone.yml

@@ -6,10 +6,12 @@ node:
   nix-daemon: 1
 
 steps:
-  #- name: check formatting
-  #  image: nixpkgs/nix:nixos-22.05
-  #  commands:
-  #    - nix run --extra-experimental-features nix-command --extra-experimental-features flakes github:nixos/nixpkgs/nixos-22.05#cargo fmt -- --check
+  - name: check formatting
+    image: nixpkgs/nix:nixos-22.05
+    environment:
+      NIX_PATH: 'nixpkgs=channel:nixos-22.05'
+    commands:
+      - nix-shell -p cargo -p rustfmt --run 'cargo fmt -- --check'
 
   - name: build
     image: nixpkgs/nix:nixos-22.05
@@ -33,6 +35,6 @@ trigger:
 
 ---
 kind: signature
-hmac: c8525b399be9c11eec7f6549242aceda6442114e3b71b587c03c074178c06bf5
+hmac: 110a818a7e2a1c48032c552a2fa482c7ef8293c3108fa23b01a738c567060dc9
 
 ...