Github predefines to IPv6 when a dual-stack exists and on IPv6-first servers but, unfortunately, it doesn't have published Internet routes at this moment via IPv6 (has the address, but doesn't route the address).
Curl and similar tools must be forcing the IPv4 mode, in order to extract whichever necessary from Github.
This is something outside of C-Servers' responsibility.
If a message appears after forcing IPv4, saying "Network is unreachable", possibly cloud-init didn't proactively configure DNSes or had an error deploying them. Check here how to correct the issue.