ci: setup cachix build

.github/workflows/cachix.yaml

@@ -1,10 +1,5 @@
-name: "Build everything and push to cachix"
+name: "Cachix build"
-on:
+on: [push]
-  workflow_dispatch:
-  schedule:
-    - cron: '0 */4 * * *'
-  push:
-    branches:
 jobs:
   build:
     runs-on: ubuntu-latest