Bump version of iohk-nix.

Steps I followed:

Find the latest revision of `iohk-nix` at:

https://github.com/input-output-hk/iohk-nix/commits/

Use that to update the revision of iohkNix in flake.lock:

```diff
diff --git a/flake.lock b/flake.lock
index 8062f9357..7d0279b23 100644
--- a/flake.lock
+++ b/flake.lock
@@ -635,7 +635,7 @@
         "narHash": "sha256-YhS7zGd6jK/QM/+wWyj0zUBZmE3HOXAL/kpJptGYIWg=",
         "owner": "input-output-hk",
         "repo": "iohk-nix",
-        "rev": "7fc7625a9ab2ba137bc70ddbc89a13d3fdb78c8b",
+        "rev": "718a10193b92f5bb2c0d2f068273a6d6d8c46034",
         "type": "github"
       },
       "original": {
```

Run:

```
nix flake lock --update-input iohkNix
```

Then:

```
nix/regenerate.sh
```