[nix] update nixpkgs revision to support llvm-15

nix/sources.json

@@ -29,10 +29,10 @@
         "homepage": "https://github.com/NixOS/nixpkgs",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "5f326e2a403e1cebaec378e72ceaf5725983376d",
-        "sha256": "01lfac9zr1sx0whkd25d3jww3y3wcbyv4dck684k774p5gfx1wjy",
+        "rev": "d231d18e4aa5e1d00f86b4f484f9e4344538e3ea",
+        "sha256": "1ks9h2m6ns6b0wbw6x4cqaqwgfac1n51v3a8vygnm7ynq34ka2l1",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/5f326e2a403e1cebaec378e72ceaf5725983376d.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/d231d18e4aa5e1d00f86b4f484f9e4344538e3ea.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "why3": {