Add the following to
substituters = https://aseipp-nix-cache.freetls.fastly.net?priority=10 https://cache.nixos.org
cache.nixos.org is the default, with priority 40.
See the reference manual.
Lower value means higher priority.
According to that the substituters are only used when called by a trusted user or in a trusted substituter list.
According to https://nixos.wiki/wiki/Maintainers:Fastly#Beta_.2B_IPv6_.2B_HTTP.2F2 the
aesipp-nix-cache is new.
It's unclear from when that info is and what the current status of this project is (as of May 2023).