'cataclysm-dda-git.overrideAttrs (_: { version = ...; src = fetchFromGitHub { ... }; })'
did not update VERSION make flag correctly.
With this change, one can override 'cataclysm-dda-git' correctly and
more easily:
cataclysm-dda-git.override { version = ...; rev = ...; sha256 = ...; }