add workflow for pushing nix flake artifacts to Cachix (#1721)

* add workflow for pushing nix flake artifacts to Cachix

* add docs on using the cachix cache from nix

* remove submodule clone from cachix workflow

* remove flake check

.github/workflows/cachix.yml

@@ -0,0 +1,24 @@
+# Publish the Nix flake outputs to Cachix
+name: Cachix
+on:
+  push: master
+
+jobs:
+  publish:
+    name: Publish Flake
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@v2
+
+    - name: Install nix
+      uses: cachix/install-nix-action@v16
+
+    - name: Authenticate with Cachix
+      uses: cachix/cachix-action@v10
+      with:
+        name: helix
+        authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+
+    - name: Build nix flake
+      run: nix build

book/src/install.md

@@ -19,7 +19,12 @@ ### NixOS
 
 A [flake](https://nixos.wiki/wiki/Flakes) containing the package is available in
 the project root. The flake can also be used to spin up a reproducible development
-shell for working on Helix.
+shell for working on Helix with `nix develop`.
+
+Flake outputs are cached for each push to master using
+[Cachix](https://www.cachix.org/). With Cachix
+[installed](https://docs.cachix.org/installation), `cachix use helix` will
+configure Nix to use cached outputs when possible.
 
 ### Arch Linux