CI: GitHub: add Optional-Nix-dev-env-main: add mvp nix-build (#95)

Work towards having enough CI testing #67.

I currently can not create the Cachix binary cache current for the project, details in #96.

So for the time being added the simple GH cache to them.

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

@@ -0,0 +1,44 @@
+name: "(Optional) Nix dev env, Linux, main"
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+  schedule:
+    - cron: "45 03 * * *"
+
+
+jobs:
+  #  2020-12-22: NOTE: This is very MVP - is should be expanded, like it is in HNix.
+  build10:
+    name: "Default"
+    runs-on: ubuntu-latest
+    continue-on-error: true
+    strategy:
+      matrix:
+        packageRoot: [ hnix-store-core, hnix-store-remote ]
+    defaults:
+      run:
+        working-directory: "./${{ matrix.packageRoot }}"
+    steps:
+    - name: "Git checkout"
+      uses: actions/checkout@v2
+      with:
+        submodules: recursive
+    #  2020-12-22: NOTE: After registering/adding Cachix - local cache should be reduced.
+    - name: "Cache of /nix/store"
+      uses: actions/cache@v2
+      with:
+        path: |
+          /nix/store
+        key: "${{ runner.os }}-Nix"
+    - name: "Install Nix"
+      uses: cachix/install-nix-action@v12
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - name: "Determined Nix-build"
+      run: nix-build ../ -A "pkgs.haskellPackages.${{ matrix.packageRoot }}"
+
+
+