Add update-all-members workflow

[vladh]

This workflow will run every Monday at 16:25 UTC, and it will attempt to fetch new JSON files for every member in members.json.

I chose this time based on these criteria:

• Inside working hours in both GMT/BST and EST/EDT, so we can fix anything that goes wrong.
• Not at the top of the hour, so avoids peak times.
• Once a week should be more than enough for data that is likely to change on the timescale of months.
• Tomorrow's Monday, so we can check that everything is working right.

Just a note: GitHub dictates that scheduled workflows can only run on main.

Also: we can always update the JSON files manually by running ./bin/update-all-members.