The CI container set `nix.settings.sandbox-fallback` but never
`nix.settings.fallback`, so a cache-miss build with an unreachable
remote build machine had no local fallback and hard-failed. A single
down or DNS-unresolvable `nix.buildMachines` host therefore turned every
fresh `nix flake check` red hive-wide, in ~30s, even for diffs that
can't affect the nix build (pure JS/markdown). Add
`nix.settings.fallback = true` so CI degrades to a slower local build
instead of failing.
86ad8bc914