[easy][devboxjson] Add nixpkgs to schema (#2498)

## Summary

TSIA

## How was it tested?

Author: mikeland73

.schema/devbox.schema.json

@@ -139,7 +139,16 @@
         },
         "env_from": {
             "type": "string"
+        },
+        "nixpkgs": {
+            "type": "object",
+            "properties": {
+                "commit": {
+                    "type": "string",
+                    "description": "The commit hash of the nixpkgs repository to use"
+                }
+            }
         }
     },
     "additionalProperties": false
-}
+}