ci: check code formats

Use a job to check the format of the sourcecode with `cargo fmt`
command. The new step does not change any code, but returns an error
when `check` does not pass with 0 issues.

.github/workflows/build.yml

@@ -274,3 +274,28 @@ jobs:
       #   with:
       #     name: 'ouch-x86_64-pc-windows-gnu'
       #     path: target\x86_64-pc-windows-gnu\release\ouch.exe
+
+  fmt:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v2
+
+      - name: Install toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: nightly
+          target: x86_64-unknown-linux-musl
+          components: rustfmt
+          override: true
+
+      - name: Check format with cargo fmt
+        uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --all -- --check