Support containerised jobs on self-hosted runners

There is a runner bug where the github.action_path and
GITHUB_ACTION_PATH variables are not equal when running in a container
on a self-hosted runner, with the former being set incorrectly. See
actions/runner#716 for details.

Fixes #1.

action.yml

@@ -14,7 +14,7 @@ runs:
   steps:
 
     - name: Lint newline
-      run: ${{ github.action_path }}/git-no-newlines
+      run: $GITHUB_ACTION_PATH/git-no-newlines
       shell: bash
       env:
         EXCLUDE_PATTERN: ${{ inputs.exclusion_pattern }}