Use universal formatting CI check

Author: benalleng

.github/workflows/format.yml

@@ -0,0 +1,18 @@
+name: Continuous integration
+
+on: [push, pull_request]
+
+jobs:
+  Format:
+    runs-on: ubuntu-latest
+    steps:
+      - name: "Checkout repo"
+        uses: actions/checkout@v4
+      - name: "Install nix"
+        uses: DeterminateSystems/determinate-nix-action@main
+      - name: "Use nix cache"
+        uses: DeterminateSystems/magic-nix-cache-action@main
+      - name: "Install treefmt"
+        run: nix profile install github:numtide/treefmt
+      - name: "Run formatting check"
+        run: treefmt --ci

.github/workflows/rust.yml

@@ -41,34 +41,6 @@ jobs:
       - name: Run tests
         run: RUST_LOG=debug bash contrib/test.sh
 
-  Format:
-    runs-on: ubuntu-latest
-    steps:
-      - name: "Checkout repo"
-        uses: actions/checkout@v4
-      - name: "Install nightly toolchain"
-        uses: dtolnay/rust-toolchain@nightly
-        with:
-          components: rustfmt
-      - name: "Use cache"
-        uses: Swatinem/rust-cache@v2
-      - name: "Run formatting check"
-        run: cargo fmt --all -- --check
-  
-  Format-FFI:
-    runs-on: ubuntu-latest
-    steps:
-      - name: "Checkout repo"
-        uses: actions/checkout@v4
-      - name: "Install nightly toolchain"
-        uses: dtolnay/rust-toolchain@nightly
-        with:
-          components: rustfmt
-      - name: "Use cache"
-        uses: Swatinem/rust-cache@v2
-      - name: "Run formatting check"
-        run: cd payjoin-ffi && cargo fmt --all -- --check
-  
   Lint-FFI:
     runs-on: ubuntu-latest
     steps: