wrapGAppsHook: Correct wrapProgram invocations

This change fixes several defects in the way `wrapGAppsHook` selected
the executable to wrap.

Previously, it would wrap any top-level files in the target `/bin` and
`/libexec` directories, including directories and non-executable
files.  In addition, it failed to wrap files in subdirectories.

Now, it uses `find` to iterate over these directory hierarchies,
selecting only executable files for wrapping.

pkgs/build-support/setup-hooks/wrap-gapps-hook.sh

@@ -36,9 +36,10 @@ wrapGAppsHook() {
   done
 
   if [ -z "$dontWrapGApps" ]; then
-    for i in $prefix/bin/* $prefix/libexec/*; do
-      echo "Wrapping app $i"
-      wrapProgram "$i" "${gappsWrapperArgs[@]}"
+    find "${prefix}/bin" "${prefix}/libexec" -type f -executable -print0 \
+    | while IFS= read -r -d '' file; do
+      echo "Wrapping program $file"
+      wrapProgram "$file" "${gappsWrapperArgs[@]}"
     done
   fi
 }