ci: split container tests into virtualenv and Nix flake environments

Refactor CI to clearly separate virtualenv-based container tests from pure Nix flake tests across all distros (arch, debian, ubuntu, fedora, centos).
Introduce dedicated test-env-nix workflow and Makefile targets, rename former container tests to test-env-virtual, and update stable pipeline dependencies.
Improve Nix reliability in containers by fixing installer permissions and explicitly validating nix availability and version during image build and tests.

.github/workflows/ci.yml

@@ -13,8 +13,11 @@ jobs:
   test-integration:
     uses: ./.github/workflows/test-integration.yml
 
-  test-container:
-    uses: ./.github/workflows/test-container.yml
+  test-env-virtual:
+    uses: ./.github/workflows/test-env-virtual.yml
+
+  test-env-nix:
+    uses: ./.github/workflows/test-env-nix.yml
 
   test-e2e:
     uses: ./.github/workflows/test-e2e.yml

.github/workflows/mark-stable.yml

@@ -14,8 +14,11 @@ jobs:
   test-integration:
     uses: ./.github/workflows/test-integration.yml
 
-  test-container:
-    uses: ./.github/workflows/test-container.yml
+  test-env-virtual:
+    uses: ./.github/workflows/test-env-virtual.yml
+
+  test-env-nix:
+    uses: ./.github/workflows/test-env-nix.yml
 
   test-e2e:
     uses: ./.github/workflows/test-e2e.yml
@@ -30,7 +33,8 @@ jobs:
     needs:
       - test-unit
       - test-integration
-      - test-container
+      - test-env-nix
+      - test-env-virtual
       - test-e2e
       - test-virgin-user
       - test-virgin-root

.github/workflows/test-env-nix.yml

@@ -0,0 +1,26 @@
+name: Test Virgin Nix (flake only)
+
+on:
+  workflow_call:
+
+jobs:
+  test-env-nix:
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+
+    strategy:
+      fail-fast: false
+      matrix:
+        distro: [arch, debian, ubuntu, fedora, centos]
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Show Docker version
+        run: docker version
+
+      - name: Nix flake-only test (${{ matrix.distro }})
+        run: |
+          set -euo pipefail
+          distro="${{ matrix.distro }}" make test-env-nix

.github/workflows/test-env-virtual.yml

@@ -4,7 +4,7 @@ on:
   workflow_call:
 
 jobs:
-  test-container:
+  test-env-virtual:
     runs-on: ubuntu-latest
     timeout-minutes: 30
     strategy:
@@ -25,4 +25,4 @@ jobs:
       - name: Run container tests (${{ matrix.distro }})
         run: |
           set -euo pipefail
-          distro="${{ matrix.distro }}" make test-container
+          distro="${{ matrix.distro }}" make test-env-virtual