update-kernel doesn’t use the apk cache

When you update the kernel and modloop using the handy update-kernel script, it doesn’t use the apk cache at /etc/apk/cache which is quite inconsiderate since in that step many linux-firmware-* packages are installed.

The actual fix would probably be as easy as adding —cache-dir /etc/apk/cache to the _apk function but I’m unsure about other implications it may have.