Enable sparse registry access using the stable configuration

Exclude versions 1.66 and 1.67, which don't support stable sparse registry.

action.yml

@@ -78,10 +78,6 @@ runs:
         if [[ ! -v CARGO_UNSTABLE_SPARSE_REGISTRY ]]; then
           echo "CARGO_UNSTABLE_SPARSE_REGISTRY=true" >> $GITHUB_ENV
         fi
-        # Enable sparse index after stabilization
-        # This causes warnings on stable 1.67, e.g., when using "cargo add"
-        # https://github.com/rust-lang/cargo/pull/11224
-        # echo "CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse" >> $GITHUB_ENV
       shell: bash
     - name: "Install Rust Problem Matcher"
       run: echo "::add-matcher::${{ github.action_path }}/rust.json"
@@ -123,6 +119,18 @@ runs:
         echo "cachekey=$(echo $DATE$HASH | head -c12)" >> $GITHUB_OUTPUT
       shell: bash
 
+    # Copied from dtolnay/rust-toolchain and adapted
+    # https://github.com/dtolnay/rust-toolchain/blob/25dc93b901a87e864900a8aec6c12e9aa794c0c3/action.yml#L100-L108
+    - name: "Enable cargo sparse registry on stable"
+      run: |
+        # except on 1.66 and 1.67, on which it is unstable
+        if [[ ! -v CARGO_REGISTRIES_CRATES_IO_PROTOCOL ]]; then
+          if echo "${{steps.versions.outputs.rustc-version}}" | not grep -q '^rustc 1\.6[67]\.'; then
+            echo "CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse" >> $GITHUB_ENV
+          fi
+        fi
+      shell: bash
+
     - name: "Setup Rust Caching"
       if: inputs.cache == 'true'
       uses: Swatinem/rust-cache@v2