config: properly configure package manager commands

There was a missing `system_dnf5_command` option.

The `dnf_command` (DNF4) is changed from /bin/dnf to /bin/dnf-3.  The
`/bin/dnf-3` script exists since ever and clearly denotes DNF4 (there
used to be two suffixes, `-2` for Python 2 and `-3` for Python 3, but
the `-3` clearly means the DNF4 written in Python).  Doing the same for
`system_dnf_command`.

The `dnf_install_command` can be safely changed to install `python3-dnf`
instead of dnf (which in the future will be provided by DNF5).

This change is especially needed for hosts that have /bin/dnf symlink
provided by dnf5.

Relates: #1084

mock/docs/site-defaults.cfg

@@ -472,12 +472,12 @@
 #       options to always use with the builddep command
 
 # DNF (or DNF4, Python) - https://github.com/rpm-software-management/dnf
-#config_opts['dnf_command'] = '/usr/bin/dnf'
+#config_opts['dnf_command'] = '/usr/bin/dnf-3'
 #config_opts['dnf_common_opts'] = ['--setopt=deltarpm=False', '--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"]}
-#config_opts['dnf_install_command'] = 'install dnf dnf-plugins-core'
+#config_opts['dnf_install_command'] = 'install python3-dnf dnf-plugins-core'
 #config_opts['dnf_disable_plugins'] = ['local', 'spacewalk', 'versionlock']
 #config_opts['dnf_builddep_opts'] = []
 

mock/py/mockbuild/config.py

@@ -294,14 +294,15 @@ def setup_default_config_opts():
     config_opts['system_yum_command'] = '/usr/bin/yum'
     config_opts['yum_install_command'] = 'install yum yum-utils'
     config_opts['yum_builddep_command'] = '/usr/bin/yum-builddep'
-    config_opts['system_dnf_command'] = '/usr/bin/dnf'
 
-    config_opts['dnf_command'] = '/usr/bin/dnf'
+    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_install_command'] = 'install dnf dnf-plugins-core'
+    config_opts['dnf_install_command'] = 'install python3-dnf dnf-plugins-core'
     config_opts['dnf_disable_plugins'] = ['local', 'spacewalk', 'versionlock']
 
     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_install_command'] = 'install dnf5 dnf5-plugins'
     config_opts['dnf5_disable_plugins'] = []