ci: adapt to nix native runner

.github/workflows/rustfmt.yml

@@ -12,17 +12,8 @@ env:
 
 jobs:
   rustfmt:
-    runs-on: ubuntu-latest
+    runs-on: native
     steps:
     - uses: actions/checkout@v2
-    - name: Show env
-      run: |
-        uname -a
-        ldd --version
-    - name: Install
-      run: |
-        curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain nightly
-        rustup toolchain install nightly --allow-downgrade -c rustfmt
-        echo "$HOME/.cargo/bin" >> $GITHUB_PATH
     - name: Check
-      run: cargo +nightly fmt -- --check
+      run: nix develop .#dev --command cargo fmt -- --check