update nix dependencies with niv-update

* changes to nix-build output:

Version changes:
[U.]  #1  gmp-with-cxx  6.2.1 -> 6.3.0
Closure size: 13 -> 13 (13 paths added, 13 paths removed, delta +0, disk usage +12.6KiB).

* changes to nix-shell output:

Version changes:
[C.]  #1  gmp           5.1.3, 6.2.1 x2 -> 5.1.3, 6.2.1, 6.3.0
[U.]  #2  gmp-with-cxx  6.2.1 x2, 6.2.1-dev -> 6.3.0 x2, 6.3.0-dev
[U.]  #3  gnu-config    2023-01-21 -> 2023-07-31
[C.]  #4  gnutls        3.7.3, 3.8.0 -> 3.7.3, 3.8.1
[U.]  #5  mesa          23.1.4 -> 23.1.5
[C.]  #6  openssl       1.1.1n, 3.0.9 x2, 3.0.9-bin, 3.0.9-dev -> 1.1.1n, 3.0.10 x2, 3.0.10-bin, 3.0.10-dev
[C.]  #7  xz            5.2.5, 5.2.5-bin, 5.4.3, 5.4.3-bin -> 5.2.5, 5.2.5-bin, 5.4.4, 5.4.4-bin
Closure size: 1147 -> 1147 (858 paths added, 858 paths removed, delta +0, disk usage +7.6MiB).

nix/sources.json

@@ -5,10 +5,10 @@
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "d680ded26da5cf104dd2735a51e88d2d8f487b4d",
-        "sha256": "0xczslr40zy1wlg0ir8mwyyn5gz22i2f9dfd0vmgnk1664v4chky",
+        "rev": "b85ed9dcbf187b909ef7964774f8847d554fab3b",
+        "sha256": "066rjaxsyzj4q9plsy8aadnyqi2kxqq1qb05k823qqmjf346f8a8",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/d680ded26da5cf104dd2735a51e88d2d8f487b4d.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/b85ed9dcbf187b909ef7964774f8847d554fab3b.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "olderIdeaNixpkgs": {