ci: use nix instead of brew for macOS workflow

Since brew has been failing frequently lately, use the nix action
instead.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

.github/workflows/macos-build.yml

@@ -1,6 +1,6 @@
 name: macOS Latest Build
-#on: [push, pull_request]
-on: workflow_dispatch
+on: [push, pull_request]
+#on: workflow_dispatch
 
 jobs:
   build:
@@ -15,14 +15,13 @@ jobs:
         uses: actions/checkout@v2
         with: 
           submodules: recursive
-
-      - name: Install Dependencies
-        run: >-
-          bash installdeps
-
+      - name: Install nix
+        uses: cachix/install-nix-action@v22
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
       - name: Configure
         run: >-
-          cmake -B build -G Ninja ${{ matrix.cmake_build }} ${{ matrix.cmake_options }}
-
+          nix-shell --command 'cmake -B build -G Ninja ${{ matrix.cmake_build }} ${{ matrix.cmake_options }}'
       - name: Build
-        run: ninja -C build
+        run: >-
+          nix-shell --command 'ninja -C build'