Fix sparse registries environment variable

Author: dtolnay

action.yml

@@ -93,7 +93,7 @@ runs:
     - run: |
         : enable Cargo sparse registry
         echo CARGO_UNSTABLE_SPARSE_REGISTRY=true >> $GITHUB_ENV
-        echo REGISTRIES_CRATES_IO_PROTOCOL=sparse >> $GITHUB_ENV
+        echo CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse >> $GITHUB_ENV
       shell: bash
 
     - run: rustc +${{steps.parse.outputs.toolchain}} --version --verbose