e2e: disable Nix sandbox for cross-distro flake build test

- Update test_nix_build_pkgmgr.py to invoke
    nix --option sandbox false build .#pkgmgr -L
  to avoid sandbox/permission issues in Debian and Ubuntu containers.
- Keeps the test logic identical across all distros while ensuring
  consistent flake build behaviour during E2E runs.

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

tests/e2e/test_nix_build_pkgmgr.py

@@ -69,7 +69,12 @@ class TestNixBuildPkgmgrAllDistros(unittest.TestCase):
             _run_cmd(["id"])
 
             # --- nix build .#pkgmgr -L ---
-            proc = _run_cmd(["nix", "build", ".#pkgmgr", "-L"])
+            proc = _run_cmd([
+                "nix",
+                "--option", "sandbox", "false",
+                "build", ".#pkgmgr",
+                "-L",
+            ])
             
             if proc.returncode != 0:
                 raise AssertionError(