Restore test matrix toolchain entry for branch protection

Author: rockbmb

.github/workflows/rust-checks.yaml

@@ -85,6 +85,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
+        toolchain: [stable]
     runs-on: ${{ matrix.os }}
     env:
       RUSTFLAGS: "-Cdebug-assertions=y"