Avoid composer lock diff action if composer.lock hasn't changed

[beto-aveiga]

Composer lock diff action takes about ~3 minutes on a minimal site.
There is no need to run this workflow if there are no composer changes, or we could avoid running it for 3 minutes (depending on the site, this may vary).
We could skip it in the mentioned scenario and use less than 1 minute.

[deviantintegral]

Good call. I think we can do this with a path filter in GitHub at least: https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore