[nix] Refactor flake

.github/workflows/test.yml

@@ -1,14 +1,22 @@
-name: "Test"
-on:
-  pull_request:
-  push:
+name: "CI"
+
+on: [push, pull_request]
+
 jobs:
-  tests:
+  build:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        emacs_version:
+          - 28.1
+          - 28.2
+          - release-snapshot
+          - snapshot
     steps:
-    - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v18
+    - uses: purcell/setup-emacs@master
       with:
-        extra_nix_config: |
-          access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-    - run: nix flake check
+        version: ${{ matrix.emacs_version }}
+
+    - uses: actions/checkout@v3
+    - name: Run tests
+      run: nix --extra-experimental-features "nix-command flakes" develop -c eldev --unstable -p -d -t -T test