Merge branch 'update-nixpkgs' into 'develop'

Update nixpkgs.

See merge request !193

nixpkgs-2105.json

 {
   "name": "release2105",
-  "url": "https://releases.nixos.org/nixos/21.05/nixos-21.05.3367.fd8a7fd07da/nixexprs.tar.xz",
-  "sha256": "12p7v805xj5as2fbdh30i0b9iwy8y24sk256rgqfqylxj1784mn8"
+  "url": "https://releases.nixos.org/nixos/21.05/nixos-21.05.3468.92609f3d9bc/nixexprs.tar.xz",
+  "sha256": "154bws59vasydyqb8kfi32fyhawqpwpn85y0sn93hz9ch0r6pgvh"
 }