sync-calendar tool breaks entries it updates on Mac OS

[tidoust]

The code already notes that the logic to select all text in an input to clear it won't work properly on Mac OS:
https://github.com/w3c/tpac-breakouts/blob/main/tools/lib/calendar.mjs#L319

The tool runs apparently fine but essentially duplicates the text throughout calendar entries it touches. If the issue cannot be fixed, the tool should detect that it's running on Mac OS and decline to update the calendar entry, so that the failures don't go unnoticed.