qtwebapp: fix sourceRoot for fetchedSourceNameDefault = "full"

Without the chnage the build fails on `master` as:

    $ nix build -f. qtwebapp --arg config ' { fetchedSourceNameDefault = "full"; }' -j1
    ...
       > unpacking source archive /nix/store/yi6b975bbh7zd7wj15dan247mkbhlg5x-QtWebApp-1.9.1-github-source
       > source root is source/QtWebApp
       > chmod: cannot access 'source/QtWebApp': No such file or directory

This happens because the `sourceRoot` hardcodes unpack source dir as
`source`. Let's derive it from `src` instead.

pkgs/by-name/qt/qtwebapp/package.nix

@@ -18,7 +18,7 @@ stdenv.mkDerivation (finalAttrs: {
 
   __structuredAttrs = true;
 
-  sourceRoot = "source/QtWebApp";
+  sourceRoot = "${finalAttrs.src.name}/QtWebApp";
 
   postPatch = ''
     cat >>QtWebApp.pro <<EOF