to fix a deprecation-warning:
"…/squeekboard/debian/cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier,
you can symlink `config` to `config.toml`"
Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/659>
It is currently not possible to build Squeekboard with only
packages that are available in Debian.
This is not likely to change until Squeekboard has been ported
to GTK4 or gtk4-layer-shell.
Until then, vendor the dependencies during the build,
so that one can still use the ".deb"-packages for testing.
Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/659>
