[ci] Update unstable nix

nix/sources.json

@@ -29,10 +29,10 @@
         "homepage": "https://github.com/NixOS/nixpkgs",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "2deb07f3ac4eeb5de1c12c4ba2911a2eb1f6ed61",
-        "sha256": "0036sv1sc4ddf8mv8f8j9ifqzl3fhvsbri4z1kppn0f1zk6jv9yi",
+        "rev": "48d63e924a2666baf37f4f14a18f19347fbd54a2",
+        "sha256": "0dcxc4yc2y5z08pmkmjws4ir0r2cbc5mha2a48bn0bk7nxc6wx8g",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/2deb07f3ac4eeb5de1c12c4ba2911a2eb1f6ed61.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/48d63e924a2666baf37f4f14a18f19347fbd54a2.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "why3": {