ci: Use stable Rust for build step. (#22)

This updates the checkout action to the current latest (v4) and
uses another action to make sure that the version of Rust being
used is the current stable version (rather than whatever GitHub
last installed).

.github/workflows/ci.yml

@@ -14,7 +14,12 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
+      - name: install stable toolchain
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: stable
+          components: rustfmt
       - name: Build
         run: cargo build --workspace --verbose
       - name: Run tests