BundleUtilities: Avoid unnecessary chmod in fixup_bundle_item

Move the permissions modification added by commit 88fed668b1 (Make
bundle items writable before fixup, 2010-09-07, v2.8.3~129^2) inside the
condition added by commit 45ed314bff (BundleUtilities: do not run
install_name_tool on scripts, 2020-07-27, v3.19.0-rc1~404^2).
There is no reason to add write permissions to a file that we are
not going to modify.

Fixes: #24424