ci: avoid GH runner bug about workspaces

see issue actions/checkout#785

Author: pthierry-ledger

action.yaml

@@ -36,5 +36,5 @@ runs:
     - name: Link toolchain dir to predictable path in container
       run: |
         source=$(dirname $TOOLCHAIN_PATH)
-        ln -s $source ${{ github.workspace}}/${{ inputs.triple }}
+        ln -s $source $$GITHUB_WORKSPACE/${{ inputs.triple }}
       shell: bash