fix(#1821): remove nix.settings.fallback from hive-ci once daemon routing confirmed

The container-level nix.settings.fallback = lib.mkForce true (added as an
interim safety net so a down remote builder degrades gracefully rather than
hard-failing CI) is rendered redundant by the host-daemon routing fix: with
NIX_REMOTE=daemon set for all systemd units in the container, every nix
build invocation routes through the host daemon, which already has its own
fallback policy. The container setting would only apply to nix invocations
that bypass the daemon entirely — a case that should not exist post-deploy.

NOTE: draft — gated on the daemon-routing change (PR that adds
NIX_REMOTE=daemon to systemd.globalEnvironment + hive-priv service env)
being deployed and confirmed working in production before merging.

nix/modules/hive-ci.nix

@@ -418,15 +418,10 @@ in
 
           # nspawn containers can't create user-namespaces, so nix
           # sandboxing always fails. Fall back to unsandboxed builds.
-          # See docs/gotchas.md.
+          # Moot once every nix invocation in the container routes
+          # through the host daemon (the daemon governs sandboxing).
+          # See docs/gotchas.md and harness-base.nix.
           nix.settings.sandbox-fallback = lib.mkForce true;
-          # Build locally when a configured remote builder is unreachable
-          # instead of hard-failing. Without this a single down/unresolvable
-          # build machine (e.g. a DNS blip on a `nix.buildMachines` host)
-          # turns every cache-miss `nix flake check` red hive-wide — CI must
-          # degrade to a slower local build, not fail. (`sandbox-fallback`
-          # above is unrelated: it only governs the sandbox, not builders.)
-          nix.settings.fallback = lib.mkForce true;
           nix.settings.experimental-features = [
             "nix-command"
             "flakes"