wiki-js: unpack into source

We effectively copy everything into `$out` (but this isn't using
`buildCommand` to allow applying custom patches). However, this had the
effect that `env-vars` was also copied into `$out` retaining a reference
to the source tarball.

Removing that reduces the closure size from 765.5M to 388.8M, i.e. by
about 50.7%.

pkgs/servers/web-apps/wiki-js/default.nix

@@ -9,6 +9,15 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-beP9k1msJjg9IQbU/CmzTodjMvUnWrLYcw0EleR1OJk=";
   };
 
+  # Unpack the tarball into a subdir. All the contents are copied into `$out`.
+  # Unpacking into the parent directory would also copy `env-vars` into `$out`
+  # in the `installPhase` which ultimately means that the package retains
+  # references to build tools and the tarball.
+  preUnpack = ''
+    mkdir source
+    cd source
+  '';
+
   sourceRoot = ".";
 
   dontBuild = true;