updateAutotoolsGnuConfigScriptsHook: Only fix files

Some package had a broken symlink named config.sub, and the script would
fail.

pkgs/build-support/setup-hooks/update-autotools-gnu-config-scripts.sh

@@ -4,7 +4,7 @@ updateAutotoolsGnuConfigScriptsPhase() {
     if [ -n "$dontUpdateAutotoolsGnuConfigScripts" ]; then return; fi
 
     for script in config.sub config.guess; do
-        for f in $(find . -name "$script"); do
+        for f in $(find . -type f -name "$script"); do
             echo "Updating Autotools / GNU config script to a newer upstream version: $f"
             cp -f "@gnu_config@/$script" "$f"
         done