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.

docs/HACKING.md

@@ -44,6 +44,24 @@ or:
 $ mkosi qemu
 ```
 
+By default, the tools from your host system are used to build the image. To have
+`mkosi` use the systemd tools from the `build/` directory, add the following to
+`mkosi.local.conf`:
+
+```conf
+[Host]
+ExtraSearchPaths=build/
+```
+
+And if you want `mkosi` to build a tools image and use the tools from there
+instead of looking for tools on the host, add the following to
+`mkosi.local.conf`:
+
+```conf
+[Host]
+ToolsTree=default
+```
+
 Every time you rerun the `mkosi` command a fresh image is built, incorporating
 all current changes you made to the project tree. To avoid having to build a new
 image all the time when iterating on a patch, add the following to