CI: Nix-dev-env-main: use Cachix & rm GitHub cache (#105)

.github/workflows/Optional-Nix-dev-env-main.yml

@@ -9,6 +9,11 @@ on:
     - cron: "45 03 * * *"
 
 
+env:
+  cachixAccount: "hnix-store"
+  CACHIX_SIGNING_KEY: ${{ secrets.CACHIX_SIGNING_KEY }}
+
+
 jobs:
   #  2020-12-22: NOTE: This is very MVP - is should be expanded, like it is in HNix.
   build10:
@@ -26,17 +31,15 @@ jobs:
       uses: actions/checkout@v2
       with:
         submodules: recursive
-    #  2020-12-22: NOTE: After registering/adding Cachix - local cache should be reduced.
-    - name: "Cache of /nix/store"
-      uses: actions/cache@v2
-      with:
-        path: |
-          /nix/store
-        key: "${{ runner.os }}-Nix"
     - name: "Install Nix"
       uses: cachix/install-nix-action@v12
       with:
         nix_path: nixpkgs=channel:nixos-unstable
+    - name: "Install Cachix"
+      uses: cachix/cachix-action@v7
+      with:
+        name: ${{ env.cachixAccount }}
+        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
     - name: "Determined Nix-build"
       run: nix-build ../ -A "haskellPackages.${{ matrix.packageRoot }}"
       env: