Run find in a cross-platform manner

[ajm188]

Managed to finally get my hands on a relevant log entry and saw:

{"level":"error","github_event_type":"pull_request","github_delivery_id":"<omitted>","error":"Failed to `find` latest docs version to generate cobradocs preview for https://github.com/vitessio/vitess/pull/14789: exit status 1\nstderr: find: unknown predicate `-sE'\n\nstdout: ","time":"2024-01-24T13:13:22Z","message":"Unexpected error handling webhook"}

So, we need to remove -sE on unix, but not on darwin.

-E before the path is replaced by -regextype posix-extended after the path.

There is no replacement for -s on non-darwin unix, so we now need to pipe to sort to sort in dictionary order.

---

Tested running this on both my laptop:

➜  vtwebsite git:(cross-platform-find) ✗ bash -c "find -E content/en/docs -maxdepth 1 -type d -regex '.*/[0-9]+.[0-9]+' | sort -d"
content/en/docs/16.0
content/en/docs/17.0
content/en/docs/18.0
content/en/docs/19.0

And on the bot:

vitess-bot:website# bash -c "find content/en/docs -regextype posix-extended -maxdepth 1 -type d -regex '.*/[0-9]+.[0-9]+' | sort -d"
content/en/docs/16.0
content/en/docs/17.0
content/en/docs/18.0
content/en/docs/19.0

I believe this will fix #63 but I will close manually after verifying.