CI/fmt: introduce separated workflow for formatting

Linting with clippy depends on the Rust version, formatting does not as we
depend on nightly now. It makes sense to split in two different files. It will
also run in parallel and are independent.

Author: dannywillems

.github/workflows/fmt.yaml

@@ -0,0 +1,19 @@
+name: Format
+
+on:
+  push:
+    branches: [ main, develop ]
+  pull_request:
+
+jobs:
+  format:
+    name: Format using nightly
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: nightly
+          components: rustfmt
+      - name: rustfmt
+        run: make check-format