Cachix caching for nix-shell GitHub Action (#1081)

Squashed commit of the following:

commit be261f9005d7f0382bd28e384542e141d47c4c35
Author: Peter Becich <[email protected]>
Date:   Wed Jun 1 20:17:02 2022 -0700

    Revert "test"

    This reverts commit 678a1ed.

commit 678a1ed
Author: Peter Becich <[email protected]>
Date:   Wed Jun 1 19:47:10 2022 -0700

    test

commit 7c8b2ee
Author: Peter Becich <[email protected]>
Date:   Wed May 25 18:55:42 2022 -0700

    use Cachix in GitHub Action

commit b99b637
Author: Peter Becich <[email protected]>
Date:   Tue May 24 00:17:53 2022 -0700

    attempt to fix Github Actions caching

commit 49f09ed
Author: Peter Becich <[email protected]>
Date:   Mon May 23 19:40:27 2022 -0700

    attempt to cache `/nix` in Nix Shell Github Action

    cachix/install-nix-action#56 (comment)

commit 5a5b310
Author: Peter Becich <[email protected]>
Date:   Mon May 23 19:35:18 2022 -0700

    attempt to use cache in Nix Shell Github Action

    https://github.com/cachix/install-nix-action#how-do-i-add-a-binary-cache

Author: peterbecich

.github/workflows/nix-shell.yml

@@ -1,13 +1,7 @@
-# https://nix.dev/tutorials/continuous-integration-github-actions
 name: "Test nix-shell"
 on:
-  push:
-    branches:
-    - '**'
-    paths-ignore: []
-  pull_request:
-    paths-ignore: []
-
+  - push
+  - pull_request
 jobs:
   nix-shell:
     runs-on: ubuntu-latest
@@ -16,4 +10,12 @@ jobs:
     - uses: cachix/install-nix-action@v16
       with:
         nix_path: nixpkgs=channel:nixos-21.11
+        extra_nix_config: |
+          trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+          substituters = https://hydra.iohk.io https://cache.nixos.org/
+    - uses: cachix/cachix-action@v10
+      with:
+        # https://nix.dev/tutorials/continuous-integration-github-actions#setting-up-github-actions
+        name: hackage-server
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
     - run: nix-shell --pure --run "cabal update && cabal build all --enable-tests"