Add crawl time to report

[tidoust]

Diff files take for granted that crawls run once per day at most. Switch to GitHub Actions means that crawl will run 4 times per day, leading to meaningless titles such as:

Diff between report from "September 12, 2019" and reference report from "September 12, 2019"
https://github.com/tidoust/reffy-reports/blob/a3607828d114095853319fd614c62bc317702945/whatwg/diffnew.md

Crawl title should include the time of the crawl to disambiguate.