action: Install dependencies in action repo checkout

action: Install dependencies in action repo checkout

Since we symlink systemd-nspawn from the build directory to /usr/bin,
if the directory that nspawn was built in is cleaned up (e.g. via
git clean), systemd-nspawn is gone as well. To make it harder to
accidentally delete systemd-nspawn, let's clone and build all the
dependencies we compile from source in the repo checkout of the
action instead of in the repo of the project that's using the mkosi
action.