Use environment files instead of set-output

Problem: We're getting the following warning in the CI:

> Warning: The `set-output` command is deprecated and will be disabled
> soon. Please upgrade to using Environment Files. For more information
> see:
> https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Solution: Stop using `set-output`, start using environment files as
demonstrated in the link above.

entrypoint.sh

@@ -49,4 +49,4 @@ if [ "$need_run" == true ]; then
   set +x
 fi
 
-echo "::set-output name=xrefcheck-path::$xrefcheck_path"
+echo "xrefcheck-path=$xrefcheck_path" >> $GITHUB_OUTPUT