chore: Migrate to new spago

This replaces `shell.nix` with `flake.nix` so we can use
`purescript-overlay` with the new `spago`. (I'm not sure all this is
needed, just adapting from configuration I've seen for other projects,
like
thomashoneyman/purescript-halogen-realworld#124)

I've run `npx spago-legacy migrate` and manually added the test
dependencies to the new `spago.yaml` file.

.github/workflows/ci.yml

@@ -1,26 +1,39 @@
 name: CI
 
-on: 
-  push: 
-    branches: main
+on:
+  push:
+    branches: [main]
   pull_request:
 
 jobs:
   build:
     runs-on: ubuntu-latest
+
     steps:
       - uses: actions/checkout@v2
 
-      - uses: thomashoneyman/setup-purescript@main
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v4
+
+      - name: Setup Nix cache
+        uses: DeterminateSystems/magic-nix-cache-action@v2
 
       - name: Cache PureScript dependencies
         uses: actions/cache@v2
         with:
-          key: ${{ runner.os }}-spago-${{ hashFiles('**/*.dhall') }}
+          key: ${{ runner.os }}-spago-${{ hashFiles('**/spago.yaml') }}
           path: |
             .spago
             output
 
-      - run: spago build
+      - name: Load Nix environment
+        run: nix develop
+
+      - name: Build source
+        run: spago build
+
+      - name: Run tests
+        run: spago test
 
-      - run: spago -x test/test.dhall test
+      - name: Check formatting
+        run: purs-tidy check src test

flake.nix

@@ -0,0 +1,29 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    purescript-overlay.url = "github:thomashoneyman/purescript-overlay";
+    purescript-overlay.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = {
+    self,
+    nixpkgs,
+    flake-utils,
+    purescript-overlay,
+  }:
+    flake-utils.lib.eachDefaultSystem (system: let
+      overlays = [purescript-overlay.overlays.default];
+      pkgs = import nixpkgs {inherit system overlays;};
+    in {
+      devShells.default = pkgs.mkShell {
+        buildInputs = with pkgs; [
+          purs
+          purs-tidy
+          purs-backend-es
+          spago-unstable
+          nodejs_20
+        ];
+      };
+    });
+}