Container nix invocations hard-failed whenever the remote builder
muede-pc2 was unreachable, while the identical build on the host
succeeded. Both go through the same host nix-daemon, so the difference
looked impossible.
The cause is that `fallback` is a client-side option: the nix client
transmits it to the daemon per connection (`tryFallback` in setOptions),
so the caller decides whether a failed remote dispatch may degrade to a
local build — even when the build itself runs on the host daemon under
NIX_REMOTE=daemon. Only genuinely daemon-side settings such as
`builders` are inherited from the host. The containers never set
`fallback`, so they took nix's default of false.
Set it in the agent-container base module and in the CI container, and
correct the hive-ci comment that claimed fallback was inherited from
the host daemon along with buildMachines and max-jobs.
Verified in an agent container: `nix fmt` fails outright on the remote
store's connection reset, while the same command with fallback enabled
reports the same connection error and then builds locally and succeeds.
65a0686297