Add E2E Nix flake build test across all distro containers

- Introduce tests/e2e/test_nix_build_pkgmgr.py to inspect the Nix environment
  and build the pkgmgr flake inside the container started by test-e2e.sh
- Run the same commands in every distro container: nix --version, sandbox
  config, id, and nix build .#pkgmgr -L
- Print stdout/stderr and assert the flake build succeeds for easier
  cross-distro Nix debugging

https://chatgpt.com/share/693aa33f-4e3c-800f-86ec-99c38a07eacb

tests/e2e/test_nix_build_pkgmgr.py

@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+"""
+E2E test to inspect the Nix environment and build the pkgmgr flake
+in *every* distro container.
+
+Commands executed inside the container (for all distros):
+
+    nix --version
+    nix show-config | grep sandbox
+    id
+    nix build .#pkgmgr -L
+
+No docker is called from here – the outer test harness
+(scripts/test/test-e2e.sh) is responsible for starting the container.
+"""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import unittest
+
+
+# Resolve project root (the repo where flake.nix lives, e.g. /src)
+PROJECT_ROOT = os.path.abspath(
+    os.path.join(os.path.dirname(__file__), "..", "..")
+)
+
+
+def _run_cmd(cmd: list[str]) -> subprocess.CompletedProcess:
+    """
+    Run a command in a subprocess, capture stdout/stderr and print them.
+
+    Does NOT raise by itself – the caller checks returncode.
+    """
+    print("\n[TEST] Running command:", " ".join(cmd))
+    proc = subprocess.run(
+        cmd,
+        text=True,
+        capture_output=True,
+    )
+    print("[STDOUT]\n", proc.stdout)
+    print("[STDERR]\n", proc.stderr)
+    print("[RETURN CODE]", proc.returncode)
+    return proc
+
+
+class TestNixBuildPkgmgrAllDistros(unittest.TestCase):
+    """
+    E2E test that runs the same Nix diagnostics + flake build
+    in all distro containers.
+    """
+
+    def test_nix_env_and_build_pkgmgr(self) -> None:
+        # Ensure we are in the project root (where flake.nix resides)
+        original_cwd = os.getcwd()
+        try:
+            os.chdir(PROJECT_ROOT)
+
+            # --- Nix version ---
+            _run_cmd(["nix", "--version"])
+
+            # --- Nix sandbox setting ---
+            _run_cmd(["sh", "-c", "nix show-config | grep sandbox || true"])
+
+            # --- Current user ---
+            _run_cmd(["id"])
+
+            # --- nix build .#pkgmgr -L ---
+            proc = _run_cmd(["nix", "build", ".#pkgmgr", "-L"])
+
+            if proc.returncode != 0:
+                raise AssertionError(
+                    "nix build .#pkgmgr -L failed inside the test container.\n"
+                    f"Exit code: {proc.returncode}\n"
+                    "See STDOUT/STDERR above for Nix diagnostics."
+                )
+
+        finally:
+            os.chdir(original_cwd)
+
+
+if __name__ == "__main__":
+    unittest.main()