ci: Use cargo-hack's --rust-version flag for msrv check

This respects rust-version field in Cargo.toml, so it removes the need
to manage MSRV in both the CI file and Cargo.toml.

Author: taiki-e

.github/workflows/ci.yml

@@ -97,11 +97,8 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest]
-        # When updating this, the reminder to update the minimum supported
-        # Rust version in Cargo.toml.
-        rust: ['1.70']
     steps:
       - uses: actions/checkout@v4
-      - name: Install Rust
-        run: rustup update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
-      - run: cargo build
+      - name: Install cargo-hack
+        uses: taiki-e/install-action@cargo-hack
+      - run: cargo hack build --rust-version