Make cargo fmt check a separate job in CI

Similar to what we did earlier with cargo-doc, this change makes the
cargo fmt invocation a separate step. In so doing the check is likely to
run much earlier in the run, resulting in a quicker experience when code
is not properly formatted.

Signed-off-by: Daniel Müller <deso@posteo.net>

Author: d-e-s-o

.github/workflows/rust.yml

@@ -57,12 +57,21 @@ jobs:
       run: cargo test --verbose --workspace --exclude runqslower -- --skip test_object --skip test_tc
     - name: Run BTF tests
       run: cd libbpf-rs && cargo test --verbose -- test_object test_tc
-    - if: ${{ matrix.rust != 'nightly' }}
-      name: Run rustfmt
-      run: cargo fmt --package libbpf-cargo libbpf-rs -- --check
     - if: ${{ matrix.rust != 'nightly' }}
       name: Run clippy
       run: cargo clippy --tests -- -D warnings
+  rustfmt:
+    name: Check code formatting
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          components: rustfmt
+          override: true
+      - run: cargo fmt --package libbpf-cargo libbpf-rs -- --check
   cargo-doc:
     name: Check documentation
     runs-on: ubuntu-latest