feat(pkgs): Add 'wallpapers' derivation

Adds a derivation that fetches my wallpapers from git.

I'm getting closer to understanding "the Nix way", i.e. that maybe
thinking about stuff like this in terms of "the wallpapers should be
in my home folder" is incorrect.

pkgs/wallpapers.nix

@@ -0,0 +1,21 @@
+# Fetch my wallpapers from git
+with import <nixpkgs> {};
+
+stdenv.mkDerivation {
+  name    = "tazjins-wallpapers-1";
+
+  src = fetchgit {
+    url = "https://git.tazj.in/tazjin/wallpapers.git";
+    rev = "3bce73b605ba5f848cb4e7cc33058a2be3952c68";
+  };
+
+  installPhase = ''
+    mkdir -p $out/share/wallpapers
+    cp -r $src/* $out/share/wallpapers
+  '';
+
+  meta = with stdenv.lib; {
+    description = "tazjin's wallpaper collection";
+    platforms = platforms.all;
+  };
+}