sane-backends: update saneBackendsGit to the latest version

For some reason, SANE suddenly stopped recognizing my scanner recently:

 | $ scanimage -L
 |
 | No scanners were identified. If you were expecting something different,
 | check that the scanner is plugged in, turned on and detected by the
 | sane-find-scanner tool (if appropriate). Please read the documentation
 | which came with this software (README, FAQ, manpages).

I was able to remedy this issue by building SANE with the latest version
of the backends package from Git, by adding the following override to
~/.nixpkgs/config.nix:

 | {
 |   packageOverrides = pkgs:
 |   {
 |     saneBackends = pkgs.saneBackendsGit;
 |   };
 | }