CI: Run semver-checks using stable Rust

Author: phil-opp

.github/workflows/build.yml

@@ -236,4 +236,4 @@ jobs:
           cache-targets: false
       - run: cargo install cargo-semver-checks --locked
       - name: Check semver
-        run: cargo semver-checks check-release
+        run: cargo +stable semver-checks check-release