use github openwrt packages mirror

[Habbie]

Right now we have a 92-job matrix. Future matrix updates might shift that number up or down a bit, but it'll be in the same ballpark. All of those jobs clone this repo from github, which probably is better than sticking it in an artifact. However, all of these jobs also clone the packages repo from git.openwrt.org. Just before I wrote this, that clone randomly failed at least once, leading to the omission of a target version/arch I happened to care about.

The relevant repo is mirrored at https://github.com/openwrt/packages/ and we should switch to it. Probably just a sed to feeds.conf, which we are already adding a line to.

[Habbie]

https://github.com/openwrt/gh-action-sdk/blob/498e39412015f1e046da1eff95ced9d8f3aac942/entrypoint.sh#L14