tools: xz: update to 5.6.4

tools: xz: update to 5.6.4

The serious liblzma backdoor vulnerability (CVE-2024-3094) has
been fixed since v5.6.2. It's time to bump this tool to the
latest version. This patch also added a new GitHub package URL.

Changelogs:
https://github.com/tukaani-project/xz/releases/tag/v5.6.2
https://github.com/tukaani-project/xz/releases/tag/v5.6.3
https://github.com/tukaani-project/xz/releases/tag/v5.6.4

Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18063
Signed-off-by: Nick Hainke <[email protected]>