build: Move cargo fmt check to nightly toolchain via new workflow

Enforcing group_imports="StdExternalCrate" requires using cargo fmt from
the nightly toolchain. Create a new workflow that runs on nightly to run
cargo fmt.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

.github/workflows/formatting.yaml

@@ -0,0 +1,32 @@
+name: Cloud Hypervisor Code Formatting
+on: [pull_request, merge_group]
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    name: Code Formatting
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - nightly
+        target:
+          - x86_64-unknown-linux-gnu
+          - aarch64-unknown-linux-musl
+    env:
+      RUSTFLAGS: -D warnings
+    steps:
+      - name: Code checkout
+        uses: actions/checkout@v4
+      - name: Install Rust toolchain (${{ matrix.rust }})
+        uses: dtolnay/rust-toolchain@stable
+        with:
+            toolchain: ${{ matrix.rust }}
+            target: ${{ matrix.target }}
+            components: rustfmt
+      - name: Formatting (rustfmt)
+        run: cargo fmt --all -- --check
+      - name: Formatting (fuzz) (rustfmt)
+        run: cargo fmt --all --manifest-path fuzz/Cargo.toml -- --check