ghcup-metadata: Fix metadata generation

There were some syntax errors in the generation script which were
preventing it from running.

I have tested this with:

```
nix shell --extra-experimental-features nix-command -f .gitlab/rel_eng -c ghcup-metadata --metadata ghcup-0.0.7.yaml --date="2024-05-27" --pipeline-id=95534 --version=9.11.20240525
```

which completed successfully.