Only checkout git repos once

[relud]

when a git repository is specified multiple times in repositories.yaml it is cloned separately for each instance. Given the size of some of these repos, and the likelihood of repeatedly referenced repos being large, we should make sure each repo is only cloned once.

[wlach]

Anything that speeds up the probe scraper job would be welcome, especially when debugging issues like https://bugzilla.mozilla.org/show_bug.cgi?id=1744585