ci: switch to dtolnay/rust-toolchain

The actions-rs/toolchain project appears dead. dtolnay's also seems more
sustainable given its simplicity, but it does enough to suit our needs.

.github/workflows/release.yml

@@ -112,11 +112,9 @@ jobs:
         ci/macos-install-packages
 
     - name: Install Rust
-      uses: actions-rs/toolchain@v1
+      uses: dtolnay/rust-toolchain@v1
       with:
         toolchain: ${{ matrix.rust }}
-        profile: minimal
-        override: true
         target: ${{ matrix.target }}
 
     - name: Use Cross