github: setup Actions with cachix

.github/workflows/cachix.yml

@@ -0,0 +1,18 @@
+name: "Build everything and push to cachix"
+on:
+  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 -E 'let pkgs = import <nixpkgs> { overlays = import ./overlays; }; in pkgs.bitwarden_rs-postgresql'