ci: switch to using '@master' dtolnay action

The `v1` tag exists but isn't really supported.

This mirrors [1]. See also [2].

[1]: 50086e74da
[2]: https://github.com/BurntSushi/bstr/pull/122#issuecomment-1201930916

.github/workflows/release.yml

@@ -112,7 +112,7 @@ jobs:
         ci/macos-install-packages
 
     - name: Install Rust
-      uses: dtolnay/rust-toolchain@v1
+      uses: dtolnay/rust-toolchain@master
       with:
         toolchain: ${{ matrix.rust }}
         target: ${{ matrix.target }}