Use getDateString in openmeteo

[rejas]

Fixes #4045

Otherwise the calculation comes to the result that its yesterday...

[KristjanESPERANTO]

Oh, this was introduced by my rewrite in #4032 – the migration from moment.format("YYYY-MM-DD") (local time) to .toISOString().split("T")[0] (UTC) was a careless mistake. I also checked envcanada.js and smhi.js, which also use toISOString(), but there it's intentional – UTC hour extraction for API URLs and timestamp storage – so they should be fine.

The new test is really nice: mocking toISOString to return a wrong date and asserting that the URL params are still correct is a good approach for this kind of regression.