[TS-596] Running `helm ssm` concurrently causes file to be empty

Hi there, I've recently encountered the issue that when multiple threads are running `helm ssm` on the same file, it can happen that the file gets empty. Maybe a synchronization is needed around [here](https://github.com/codacy/helm-ssm/blob/master/internal/template.go#L27). I'd be happy to open a PR for this. 