p7zip: fix src URL for debian gitlab move (#41769)

Another broken URL related to: https://github.com/NixOS/nixpkgs/issues/39927

Note that the patch file has legitimately changed, because ~4 months ago Debian
replaced their CVE security fix with a newer version that fixes some additional
bugs: https://salsa.debian.org/debian/p7zip/commit/d6fd3b37345489ec2907fcf70aabf0c754f5371f