There's a closed pull request on the void-packages repository

update_check: also work for subpackages
https://github.com/void-linux/void-packages/pull/36190

Description:
Currently `./xbps-src update-check` fails for many subpackages if the $pkgname or $pattern is not explicitly set in the update file, because it uses the subpackages $pkgname.

Example:
```
$ ./xbps-src update-check alsa-plugins-pulseaudio
NO VERSION found for alsa-plugins-pulseaudio
```

This PR tries to fix that by using $sourcepkg as the $pkgname.

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