flake: Update Nix to 2.9.1

NOTE: I'm well-aware that we have to be careful with this to avoid new
regressions on hydra.nixos.org, so this should only be merged after
extensive testing from more people.

Motivation: I updated Nix in my deployment to 2.9.1 and decided to also
update Hydra in one go (and compile it against the newer Nix). Given
that this also updates the C++ code in `hydra-{queue-runner,eval-jobs}`
this patch might become useful in the future though.

src/hydra-queue-runner/queue-monitor.cc

@@ -460,7 +460,7 @@ Step::ptr State::createStep(ref<Store> destStore,
     step->parsedDrv = std::make_unique<ParsedDerivation>(drvPath, *step->drv);
 
     step->preferLocalBuild = step->parsedDrv->willBuildLocally(*localStore);
-    step->isDeterministic = get(step->drv->env, "isDetermistic").value_or("0") == "1";
+    step->isDeterministic = getOr(step->drv->env, "isDetermistic", "0") == "1";
 
     step->systemType = step->drv->platform;
     {