Update build system and action to build .apk package

[ArtikusHG]

Fixes #8

This was much easier than I expected! These two simple changes were all that was needed to build a functional .apk package for OpenWRT 25.12. However, there's a slight problem - installation from the web interface currently seems impossible, as from what I understand, from now on, all OpenWRT packages need to be signed - and that happens at the repo level, not at the package level. So there's two possible options:

1. The only way to install the package from now on is via the command line: apk add --allow-untrusted path-to-hapt-package.apk
2. You'll need to create a repo using GitHub Pages, sign the package index and serve the new package from there. To be honest, this seems convoluted, and I'd much rather either open an issue to ask the OpenWRT devs to optionally allow --allow-untrusted from the web interface, or add to the readme that the only supported way of installing the package is via the command line.

Other than that small caveat, everything seems to work! Thanks for the amazing project, hapt has been the only 100% reliable way to detect when I'm away so far, and it makes my life a lot better :)

EDIT: it seems like other, even really popular OpenWRT projects aren't bothering with hosting their own repo, and instead just telling users to use --allow-untrusted, for example: https://github.com/Openwrt-Passwall/openwrt-passwall2. With that in mind, I'll update the readme too