ExternalProject: Also retry curl download on timeout error

Greg Thainrequested to merge
gthain/cmake:retry-file-download-on-timeout into master

Since !5034 (merged) 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

Edited by Brad King

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