Failed to init: Unable to find sdk io.elementary.Sdk version 6

When I run `flatpak-builder build org.small-tech.hello-world.yml --user --install --force-clean`, I get the following error:

```
error: io.elementary.Sdk/x86_64/6 not installed
Failed to init: Unable to find sdk io.elementary.Sdk version 6
```

`apt search elementary-sdk` gives me:

```
elementary-sdk/focal,now 1.508+r576~ubuntu6.1 amd64 [installed]
elementary Developer Kit
```

My _org.small-tech.hello-world.yml_ file contents are:

```yml
app-id: org.small-tech.hello-world

runtime: io.elementary.Platform
runtime-version: '6'
sdk: io.elementary.Sdk

command: org.small-tech.hello-world

finish-args:
- '--share=ipc'
- '--socket=fallback-x11'
- '--socket=wayland'

modules:
- name: hello-world
buildsystem: meson
sources:
- type: dir
path: .
```