ExternalProject: Also retry curl download on timeout error

Since commit f24e34975a (ExternalProject: retry download on recoverable
errors, 2020-08-08, v3.19.0-rc1~298^2~1) ExternalProject has a list of
curl errors which trigger a retry today: 6, 7, 8 and 15.

Add curl error 28.  It is a "timeout error", and should also trigger a
retry attempt at a download of an external project.  The complete list
of curl errors is https://curl.se/libcurl/c/libcurl-errors.html