Building a single package doesn't create `external-kits/external-kit-metadata.json`

## What's the problem?

A single package is built in a clean `core-kit` fork by running:

```bash
PACKAGE=kernel-6_1 make twolier build-package
```

## What really happened?

The build fails with this error:

```
Unable to instantiate the builder: Failed to create build arguments due to a dependency error: Failed to read external kit metadata file '/build/external-kits/external-kit-metadata.json': No such file or directory (os error 2)
Error while executing command, exit code: 101
```

## Workaround

Run `make ARCH=<>` and `CTRL+C` the build so that the missing file is generated.

Start by reproducing the clean-fork command `PACKAGE=kernel-6_1 make twolier build-package` and compare it with the `make ARCH=<>` workaround. Trace where `build/external-kits/external-kit-metadata.json` is expected and generated. Done means a single-package build creates the metadata file and completes without interrupting another build.

Written by the indexing model from the issue text.