Add basic GitHub Action for Nix Flake (#1246)

.github/workflows/nix-flake.yml

@@ -0,0 +1,26 @@
+name: Nix Flake
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+    container: ${{ matrix.image }}
+    strategy:
+      matrix:
+        include:
+          - os: ubuntu-latest
+          - os: macOS-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - name: "Check `nix develop` shell can open"
+        run: nix develop --command "echo"
+      # this Flake has no build target
+      # - run: nix build
+      - run: nix flake check

README.md

@@ -3,6 +3,7 @@
 [![npm](https://img.shields.io/npm/v/spago.svg)][spago-npm]
 [![Latest release](https://img.shields.io/github/v/release/purescript/spago.svg)](https://github.com/purescript/spago/releases)
 [![build](https://github.com/purescript/spago/actions/workflows/build.yml/badge.svg)](https://github.com/purescript/spago/actions/workflows/build.yml)
+[![nix-flake](https://github.com/purescript/spago/actions/workflows/nix-flake.yml/badge.svg)](https://github.com/purescript/spago/actions/workflows/nix-flake.yml)
 [![Maintainer: f-f](https://img.shields.io/badge/maintainer-f%2d-f-teal.svg)](http://github.com/f-f)
 
 _(IPA: /ˈspaɡo/)_