Exclude testscripts from Flake build

[adamdicarlo]

This stops including a potentially-colliding 'updater' binary in the derivation.

Summary

When I tried to install Zen browser on my NixOS system via a flake (zen-browser.url = "github:MarceColl/zen-browser-flake";), I got a build error saying that zen-browser included a file that collides with a devbox file:

error: builder for '/nix/store/8fy82p10qwd7z5aj4sraq9k88c62nz55-home-manager-path.drv' failed with exit code 25;
       last 1 log lines:
       > error: collision between `/nix/store/915ll44ksnhy369wyqlm8ys7ylxzmi89-devbox-0.13.6-0bc66cb/bin/updater' and `/nix/store/s2bz22csj1a8yjh3n3i1h67s5zhrh1g2-zen-browser-1.0.1-a.6/bin/updater'

My guess is that the zen-browser flake is also at fault (also errantly including a binary named updater) but this is where I looked (and figured out how to fix) first.

How was it tested?

Re-ran the build via nix build . and verified that ./result/bin/ now contains only devbox. (It used to also contain a 1.7MB file called updater.)

[Lagoja]

Good catch, I think this may be affecting some other packages as well. I thought we were already avoiding this by specifying the subpackage, but this looks more robust/explicit