Try to fix CI for semver-check.

Author: ehuss

.github/workflows/main.yml

@@ -92,7 +92,8 @@ jobs:
     - run: rustup update nightly && rustup default nightly
     - run: rustup component add rust-docs
     - run: ci/validate-man.sh
-    - run: cd src/doc/semver-check && cargo run
+    # This requires rustfmt, use stable.
+    - run: cd src/doc/semver-check && cargo +stable run
     - run: |
         mkdir mdbook
         curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.3.7/mdbook-v0.3.7-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook