Fix action to make it work on custom containers;

For further information, see: <actions/runner#716 (comment)>

Author: joergdw

action.yml

@@ -18,7 +18,7 @@ branding:
 runs:
   using: 'composite'
   steps:
-    - run : ${{ github.action_path }}/install-nix.sh
+    - run : ${GITHUB_ACTION_PATH}/install-nix.sh
       shell: bash
       env:
         INPUT_EXTRA_NIX_CONFIG: ${{ inputs.extra_nix_config }}