mkosi: Disable BuildSourcesEphemeral=

mkosi: Disable BuildSourcesEphemeral=

Let's stop using BuildSourcesEphemeral= and instead make sure we don't
generate any auxiliary files during the mkosi build process.

We achieve this through a combination of trap to remove any new files
we create and bind mounts from /tmp over existing files whenever we need
to modify an existing file.

We also add a CI step to ensure we don't regress