Use 'allow_vendor_change=yes' with DNF5

The `allow_vendor_change=no` option is going to be set in DNF5 as
default: rpm-software-management/dnf5#712

With Mock though, it is very common to override the distro default
package with a custom one (newer or fixed one?).  And blindly trying to
keep the Vendor tag unchanged for the overridden packages would be very
complicated, without any real benefits.

Note `allow_vendor_change=no` only affects the cases when some package
in the buildroot/bootstrap needs to be _updated_ while _typically_ all
the packages are only _installed_ at the beginning and are already
up2date.  This is therefore mainly related to the --chain option, or
cases like `bootstrap_image_ready=False`, `update_before_build`, the
feature in rpm-software-management#1159, etc.

Fixes: rpm-software-management#1155

mock/py/mockbuild/config.py

@@ -306,7 +306,7 @@ def setup_default_config_opts():
 
     config_opts['dnf5_command'] = '/usr/bin/dnf5'
     config_opts['system_dnf5_command'] = '/usr/bin/dnf5'
-    config_opts['dnf5_common_opts'] = ['--setopt=deltarpm=False', '--allowerasing']
+    config_opts['dnf5_common_opts'] = ['--setopt=deltarpm=False', '--setopt=allow_vendor_change=yes', '--allowerasing']
     config_opts['dnf5_install_command'] = 'install dnf5 dnf5-plugins'
     config_opts['dnf5_disable_plugins'] = []
     # No --allowerasing with remove, per