zypper has some new rather questionable userspace level permission
checking that blows ups completely when operating as root on an
cache directory owned by a non-root user, so let's build the tools
tree and set up meson as root to avoid the issue.
(https://github.com/openSUSE/libzypp/issues/603)
Also drop a leftover debug message from coverage.yml while we're at
it.
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].
```
Daan De Meyer
Matteo Croce