We still do mirror the patch (I think), as `nix-build -Q -A p7zip.patches`
downloaded it on my machine. I verified that only the diff headers differ;
it's still better to have another working download and Fedora's URLs are
less likely to disappear than Debian's.
/cc #27075.
The debian source for this patch file has gone away, rendering this derivation
unbuildable from scratch.
This change updates the URL to a src.fedoraproject.org location that is still
serving a p7zip patch. This file is not the same bytewise, so I'm also updating
the hash; I didn't manage to find a location still serving a file with the
original hash, and my best guess is that this one is functionally equivalent.
