Install cargo-msrv using stable toolchain. (#1215)

When running the CI check "msrv", we install the cargo-msrv command
using the stable Rust toolchain because it sometimes requires a higher
Rust version than our chosen version in the file `rust-toolchain`.

.github/workflows/cargo-msrv.yml

@@ -24,7 +24,8 @@ jobs:
       - run: cargo --version
 
       - name: Install cargo-msrv
-        run: cargo install cargo-msrv
+        # The cargo-msrv tool sometimes requires a higher Rust version than our current rust-toolchain.
+        run: cargo +stable install cargo-msrv
       # Verify the MSRV defined in Cargo.toml
       - name: Verify MSRV
         run: cargo msrv verify