fix(ci): update aports Git repo URL

[JohnTitor]

Description

The aports repo now returns:

[54](https://github.com/rust-lang/libc/actions/runs/21417799907/job/61670614213?pr=4946#step:7:2455)
14.37 + git clone -n --depth=1 --filter=tree:0 -b 3.20-stable https://gitlab.alpinelinux.org/alpine/aports
14.37 Cloning into 'aports'...
14.70 fatal: unable to access 'https://gitlab.alpinelinux.org/alpine/aports/': The requested URL returned error: 418

418 is unclear but auth or something else might be involved here.
This replaces it with a mirror URL as it works fine fortunately.

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI