ci: disabled scheduled build

Since we now pin nixpkgs to the same as the flake input, builds are
deterministic from run to run and there's no point running when the
version hasn't changed.

.github/workflows/build.yml

@@ -2,11 +2,6 @@ name: "Build and populate cache"
 on:
   pull_request:
   push:
-  schedule:
-    # rebuild everyday at 6:50
-    # TIP: Choose a random time here so not all repositories are build at once:
-    # https://www.random.org/clock-times/?num=1&earliest=01%3A00&latest=08%3A00&interval=5&format=html&rnd=new
-    - cron:  '50 6 * * *'
 jobs:
   tests:
     strategy: