Update GitHub Actions CI (#47)

The following updates are performed:
* update actions/checkout to v3
* replace unmaintained actions-rs/toolchain by dtolnay/rust-toolchain

.github/workflows/rustfmt.yml

@@ -7,12 +7,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
-      - uses: actions-rs/toolchain@v1
+      - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: stable
-          profile: minimal
           components: clippy, rustfmt
-          override: true
       - name: Run rustfmt
         run: make format-check