Use 'allow_vendor_change=yes' with DNF and DNF5

The `allow_vendor_change=no` option is going to be set in DNF5 as
default: rpm-software-management/dnf5#712
But it is also set by openSUSE with DNF4.

With Mock though, is very common to override the distro default package
with a custom one (newer or fixed packages?).  And blindly trying to
keep the Vendor tag unchanged for the overridden packages would be very
complicated task, without 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/docs/site-defaults.cfg

@@ -487,7 +487,7 @@
 
 # DNF (or DNF4, Python) - https://github.com/rpm-software-management/dnf
 #config_opts['dnf_command'] = '/usr/bin/dnf-3'
-#config_opts['dnf_common_opts'] = ['--setopt=deltarpm=False', '--allowerasing']
+#config_opts['dnf_common_opts'] = ['--setopt=deltarpm=False', '--setopt=allow_vendor_change=yes', '--allowerasing']
 # DF5 sub-command 'builddep' doesn't support the '--allowerasing' option:
 # https://github.com/rpm-software-management/dnf5/issues/461
 #config_opts["dnf5_avoid_opts"] = {"builddep": ["--allowerasing"]}
@@ -499,7 +499,7 @@
 # Known issue with DNF5: --forcearch is not supported by DNF5
 # https://github.com/rpm-software-management/dnf5/issues/112
 #config_opts['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'] = []
 

mock/py/mockbuild/config.py

@@ -299,14 +299,14 @@ def setup_default_config_opts():
 
     config_opts['dnf_command'] = '/usr/bin/dnf-3'
     config_opts['system_dnf_command'] = '/usr/bin/dnf-3'
-    config_opts['dnf_common_opts'] = ['--setopt=deltarpm=False', '--allowerasing']
+    config_opts['dnf_common_opts'] = ['--setopt=deltarpm=False', '--setopt=allow_vendor_change=yes', '--allowerasing']
     config_opts['dnf_install_command'] = 'install python3-dnf python3-dnf-plugins-core'
     config_opts['dnf_disable_plugins'] = ['local', 'spacewalk', 'versionlock']
     config_opts["dnf_avoid_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