Delete stable sparse registry env variable

Fixes #56.

Author: dtolnay

action.yml

@@ -91,9 +91,8 @@ runs:
       shell: bash
 
     - run: |
-        : enable Cargo sparse registry
+        : enable Cargo sparse registry (ignored by stable cargo)
         echo CARGO_UNSTABLE_SPARSE_REGISTRY=true >> $GITHUB_ENV
-        echo CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse >> $GITHUB_ENV
       shell: bash
 
     - run: rustc +${{steps.parse.outputs.toolchain}} --version --verbose