codecov action needs token

[sbfnk]

see https://docs.codecov.com/docs/adding-the-codecov-token

Otherwise results don't get uploaded. This may explain why #1010 slipped through the net although the else statement for non-integer forecasts never got visited.

[nikosbosse]

oh! who is the token holder? Do we have a token? Am I the token?

[sbfnk]

There is a token already, it just needs to be used (#1013 will ensure that)