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.

scripts/test/test-env-nix.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+IMAGE="package-manager-test-${distro}"
+
+echo "============================================================"
+echo ">>> Running Nix flake-only test in ${distro} container"
+echo ">>> Image: ${IMAGE}"
+echo "============================================================"
+
+docker run --rm \
+  -v "$(pwd):/src" \
+  -v "pkgmgr_nix_store_${distro}:/nix" \
+  -v "pkgmgr_nix_cache_${distro}:/root/.cache/nix" \
+  --workdir /src \
+  -e PKGMGR_DEV=0 \
+  "${IMAGE}" \
+  bash -lc '
+    set -euo pipefail
+
+    if command -v git >/dev/null 2>&1; then
+      git config --global --add safe.directory /src || true
+      git config --global --add safe.directory /src/.git || true
+      git config --global --add safe.directory "*" || true
+    fi
+
+    echo ">>> preflight: nix must exist in image"
+    if ! command -v nix >/dev/null 2>&1; then
+      echo "NO_NIX"
+      echo "ERROR: nix not found in image '\'''"${IMAGE}"''\'' (distro='"${distro}"')"
+      echo "HINT: Ensure Nix is installed during image build for this distro."
+      exit 1
+    fi
+
+    echo ">>> nix version"
+    nix --version
+
+    echo ">>> nix flake show"
+    nix flake show . --no-write-lock-file >/dev/null
+
+    echo ">>> nix build .#default"
+    nix build .#default --no-link --no-write-lock-file
+
+    echo ">>> nix run .#pkgmgr -- --help"
+    nix run .#pkgmgr -- --help --no-write-lock-file
+
+    echo ">>> OK: Nix flake-only test succeeded."
+  '