Add 1.6.6 test depot and CI matrix

Author: thomasjm

.github/workflows/ci.yml

@@ -5,17 +5,34 @@ on:
   push:
 
 jobs:
-  tests:
+  flake-check:
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
     steps:
     - uses: actions/checkout@v3
     - uses: cachix/install-nix-action@v18
       with:
         extra_nix_config: |
           access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-
     - run: nix flake check
 
+  tests:
+    name: ${{ matrix.os }} / Julia ${{ matrix.test_depot }}
+    runs-on: ${matrix.os}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest] # macos-latest
+        test_depot:
+        - "1.8.3"
+        - "1.6.6"
+    steps:
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v18
+      with:
+        extra_nix_config: |
+          access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
     - run: |
-        cd test_depots/1.8.3
+        cd test_depots/${{matrix.test_depot}}
         nix run ..