Let's enable usage of a tools tree by default to simplify the setup
for new contributors and save them from having to install or upgrade
a bunch of extra tools to get mkosi working as expected.
Move some config option in the right section, fixes the following warning:
```
mkosi.conf: Setting Credentials should be configured in [Runtime], not [Host].
mkosi.conf: Setting RuntimeBuildSources should be configured in [Runtime], not [Host].
mkosi.conf: Setting RuntimeScratch should be configured in [Runtime], not [Host].
mkosi.conf: Setting QemuSmp should be configured in [Runtime], not [Host].
mkosi.conf: Setting QemuSwtpm should be configured in [Runtime], not [Host].
mkosi.conf: Setting QemuVsock should be configured in [Runtime], not [Host].
mkosi.conf: Setting QemuKvm should be configured in [Runtime], not [Host].
```