add volume mount for nix

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

.github/workflows/flake-health-checks.yml

@@ -8,10 +8,11 @@ on:
 jobs:
     health-check:
         name: "Perform Nix flake checks"
-        runs-on: ${{ matrix.os }}
-        strategy:
-            matrix:
-                os: [ubuntu-latest]
+        runs-on: ubuntu-latest
+        container:
+            image: ubuntu:latest
+            volumes:
+                - "/nix:/nix"
         steps:
             - uses: DeterminateSystems/nix-installer-action@main
             - name: Setup Attic cache
@@ -26,10 +27,11 @@ jobs:
             - run: nix ./utils/attic-push.bash
     build-checks:
         name: "Build nix outputs"
-        runs-on: ${{ matrix.os }}
-        strategy:
-            matrix:
-                os: [ubuntu-latest]
+        runs-on: ubuntu-latest
+        container:
+            image: ubuntu:latest
+            volumes:
+                - "/nix:/nix"
         steps:
             - uses: DeterminateSystems/nix-installer-action@main
             - name: Setup Attic cache