install.sh: retry failed downloads (#925)
install.sh: retry failed downloads (#925)

Occasionally, downloading the Apex binary fails with a "Empty reply"
error from Github. Since the install script is only issuing GET
requests these should be safe to retry, and we can increase download
reliability by doing so.

The `--retry` flag was added in version 7.12.3, released in December
2004, so it should be available on most systems that are downloading
Apex.

Fixes #924.
apex/apex,apex/apex,apex/apex,apex/apex,apex/apex,apex/apex,apex/apex