overlays: use nixpkgs-unstable for few overlays I had

.github/workflows/cachix.yml

@@ -1,21 +0,0 @@
-name: "Build everything and push to cachix"
-on:
-  workflow_dispatch:
-  schedule:
-    - cron: '0 */4 * * *'
-  push:
-    branches:
-      - main
-jobs:
-  build:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v2
-    - uses: cachix/install-nix-action@v12
-      with:
-        nix_path: nixpkgs=channel:nixos-20.09-small
-    - uses: cachix/cachix-action@v8
-      with:
-        name: alarsyo
-        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-    - run: nix-build ./overlays/build-me.nix