Modify xpath to account for changed GH html on file lists

[nmdefries]

This morning, the forecast eval pipeline started getting

1. error in evaluating the argument 'x' in selecting a method for function 'as_date': error in evaluating the argument 'x' in selecting a method for function 'as_date': subscript out of bounds

for every forecaster requested. There have been no code changes on our end for the last several months, which implies a problem with one of the APIs or other non-package dependencies we use.

The errors are coming from lubridate attempting to parse dates from data filenames but there not being any dates to parse. It appears that the html of GitHub pages changed slightly so that the existing xpath argument doesn't work (no filenames are found).

This PR modifies the xpath so that we can read the list of files again.

[nmdefries]

@dajmcdon In discussion with @brookslogan, who had the same dates problem in the forecasting plotting pipeline, I realized that since the GitHub html change is outside of the evalcast, all evalcast versions/branches that are currently in use need the same fix applied.