p7zip: add setup hook

pkgs/tools/archivers/p7zip/default.nix

@@ -25,6 +25,8 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  setupHook = ./setup-hook.sh;
+
   meta = {
     homepage = http://p7zip.sourceforge.net/;
     description = "A port of the 7-zip archiver";

pkgs/tools/archivers/p7zip/setup-hook.sh

@@ -0,0 +1,5 @@
+unpackCmdHooks+=(_try7zip)
+_try7zip() {
+    if ! [[ "$curSrc" =~ \.7z$ ]]; then return 1; fi
+    7z x "$curSrc"
+}