ci: build as much as possible

By default GitHub Actions cancels all the jobs in the build matrix if
any one of them fails. We actually want to keep going to get some
advancement on the other channels.

.github/workflows/nix.yml

@@ -11,6 +11,8 @@ on:
 jobs:
   build:
     strategy:
+      # Keep building the other channels if one fails
+      fail-fast: false
       matrix:
         channel:
           - nixos-unstable