apt-utils: fix hard-coded GHPROXY_ADDRESS

[amazingfate]

Description

Download url of base-files.json is hard-coded to ghp.ci when GITHUB_MIRROR is set to ghproxy. But now ghp.ci is blocked. Users can use other github proxy by setting GHPROXY_ADDRESS.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• ./compile.sh BOARD=rock-5b BRANCH=vendor BUILD_MINIMAL=no DEB_COMPRESS=xz KERNEL_CONFIGURE=no RELEASE=noble GITHUB_MIRROR=ghproxy GHPROXY_ADDRESS=gitboost.haguro.top KERNEL_GIT=shallow BUILD_DESKTOP=yes DESKTOP_APPGROUPS_SELECTED= DESKTOP_ENVIRONMENT=gnome DESKTOP_ENVIRONMENT_CONFIG_NAME=config_base

Checklist:

Please delete options that are not relevant.

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules