mkosi: Streamline running the integration tests without building systemd

mkosi: Streamline running the integration tests without building systemd

Let's document in detail how to build the integration test image and run
the integration tests without building systemd. To streamline the process,
we stop automatically using binaries from build/ when invoking mkosi directly
and don't automatically use a tools tree anymore if systemd on the host is too
old. Instead, we document these options in HACKING.md and change the mkosi meson
target to automatically use the current build directory as an extra binary search
path for mkosi.