mkosi: Add extra tools tree packages required to run integration tests

mkosi: Add extra tools tree packages required to run integration tests

With https://github.com/systemd/mkosi/pull/3164, we'll be able to run
arbitrary commands in the mkosi sandbox, which has /usr from the tools
tree if one is configured. Let's add the required packages to be able to
run meson to setup the integration tests. This allows running the integration
tests without having to install meson or other build dependencies on the
host system.

"""
mkosi sandbox meson setup build
mkosi sandbox meson compile -C build mkosi
mkosi sandbox env SYSTEMD_INTEGRATION_TESTS=1 meson test -C build ...
"""