Remove use of actions-rs/toolchain@v1 in CI

Before this change, GitHub Actions would warn that actions-rs/toolchain@v1
uses deprecated functionality:

  The following actions uses node12 which is deprecated and will be 
  forced to run on node16: actions-rs/toolchain@v1. For more info: 
  https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

and:

  The `set-output` command is deprecated and will be disabled soon. 
  Please upgrade to using Environment Files. For more information see: 
  https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Unfortunately, the development of this action is abandoned (the repo
has not received any updates in the last 3 years and the author is
unresponsive: actions-rs/toolchain#219).

Fix the warning by manually invoking rustup commands since it turns
out that GitHub Actions now bundle it in their runners by default.

.github/workflows/rust.yml

@@ -51,10 +51,9 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Install Rust ${{ env.rust_min }}
-      uses: actions-rs/toolchain@v1
-      with:
-        toolchain: ${{ env.rust_min }}
-        override: true
+      run: |
+        rustup install ${{ env.rust_min }}
+        rustup override set ${{ env.rust_min }}
     - name: Print Rust version
       run: rustc --version
     - name: MSRV cargo check with features