Don't allow configuring distribution, release and mirror in tools tree config

Don't allow configuring distribution, release and mirror in tools tree config

config_default_repository_key_check() depends on ToolsTreeDistribution=
to select the right default. If the tools tree distribution is configured
in mkosi.tools.conf, we're unable to select the right default for
RepositoryKeyCheck= for the main image. Until we figure out a decent solution
for this, let's insist that the tools tree distribution is configured via
ToolsTreeDistribution=.

Similarly, selecting the default tools tree mirror depends on the selected
target distribution and mirror, so we insist on configuring the tools tree
mirror and release via ToolsTreeMirror= and ToolsTreeRelease= for now.