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