update nix dependencies with niv-update

* changes to nix-build output:

No version or selection state changes.
Closure size: 26 -> 26 (2 paths added, 2 paths removed, delta +0, disk usage +0B).

* changes to nix-shell output:

Version changes:
[C.]    nix  2.16.3, 2.16.3-man -> 2.16.3, 2.16.3-man, 2.18.2, 2.18.2-man
Closure size: 1176 -> 1178 (10 paths added, 8 paths removed, delta +2, disk usage +12.5MiB).

nix/sources.json

@@ -5,10 +5,10 @@
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "44d0940ea560dee511026a53f0e2e2cde489b4d4",
-        "sha256": "0xxbzvrr7kylvjn2yj0nnrm1qndx8rq99mlkk0ghvy364y5c5pv0",
+        "rev": "d8fe5e6c92d0d190646fb9f1056741a229980089",
+        "sha256": "0jd6x1qaggxklah856zx86dxwy4j17swv4df52njcn3ln410bic8",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/44d0940ea560dee511026a53f0e2e2cde489b4d4.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/d8fe5e6c92d0d190646fb9f1056741a229980089.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "olderIdeaNixpkgs": {