ci: setup cachix build

.github/workflows/cachix.yaml

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