Nix should be more tolerate to network outage during fetch from caches #13792
Description
Describe the bug
nix build ... fails, if nix unable to resolve cache/substituter hostname due network outage
Steps To Reproduce
Disable DNS resolving (by remove /etc/resolv.conf for example), try a nixos-rebuild or nix build
Expected behavior
skip file download (treat as timeout or 404).
Metadata
nix (Nix) 2.28.4
Additional context
Just break my nixos based router (due pdns-recursor update), and unabled to re-deploy config fixup, both nixos-rebuild and colmena starts to fail due nix build failure. (--option substutute false helps)
Checklist
- checked latest Nix manual (source)
- checked open bug issues and pull requests for possible duplicates
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