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.

flake.lock

@@ -39,16 +39,18 @@
         "nixpkgs-regression": "nixpkgs-regression"
       },
       "locked": {
-        "lastModified": 1649172203,
-        "narHash": "sha256-Q3nYaXqbseDOvZrlePKeIrx0/KzqyrtNpxHIUbtFHuI=",
+        "lastModified": 1654014617,
+        "narHash": "sha256-qNL3lQPBsnStkru3j1ajN/H+knXI+X3dku8/dBfSw3g=",
         "owner": "NixOS",
         "repo": "nix",
-        "rev": "5fe4fe823c193cbb7bfa05a468de91eeab09058d",
+        "rev": "624e38aa43f304fbb78b4779172809add042b513",
         "type": "github"
       },
       "original": {
-        "id": "nix",
-        "type": "indirect"
+        "owner": "NixOS",
+        "ref": "2.9.1",
+        "repo": "nix",
+        "type": "github"
       }
     },
     "nixpkgs": {