github: pin to specific rust toolchain version

Stable will upgrade our version, which means that CI can spontaneously
break.

.github/workflows/build-and-test.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Install protoc
         run: sudo apt install -y protobuf-compiler
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@1.76.0
       - name: run check
         run: make check