yices: 2.6.3 -> 2.6.2
yices: 2.6.3 -> 2.6.2

2.6.3 is not a stable tag (and is really a 2.6.4 in disguise).
Failure is noticed by vbgl in https://github.com/NixOS/nixpkgs/pull/141241#issuecomment-942103854

The change downgrades from unstable to latest stable release and
adds a check for file presence to prevent upgrade that silently
breaks symlink.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs