Fix msrv: Run msrv checks with minimal versions

Since it is possible to for dependencies to bump MSRV in patch release, msrv checking should use the minimal versions supported by flate2.

Users cares deeply about msrv can also pin them to the minimal versions to maintain their current msrv.

This would have also help discover bugs in #424

.github/workflows/main.yml

@@ -84,6 +84,9 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install Rust (rustup)
-        run: rustup update ${version} --no-self-update && rustup default ${version}
+        run: |
+          rustup toolchain install ${version} nightly --profile minimal --no-self-update
+          rustup default ${version}
+          cargo +nightly update -Zminimal-versions
         shell: bash
       - run: cargo build