We only support running/testing through the runner executable from now on.

Set a default target in a `.cargo/config` file instead.

…rn non-zero exit code

Move kernel package location code into a new public function.

The `cargo bootimage` command only deals with binaries, so we don't need to try to derive the manifest path for test or doctest executables.

For doctests it's not possible to derive the manifest path from the executable path and the cargo metadata since doctests live under temp and all use the same name ('rust-out'). Fortunately, cargo sets a `CARGO_MANIFEST_DIR` environment variable for runner executables that points to the manifest path of the executable (which can be different from the root manifest path for workpace projects.)