test: isolate Nix store/cache per distro to fix cross-distro manifest conflicts

- Replace shared Nix volumes with distro-specific volumes
  (pkgmgr_nix_store_<distro>, pkgmgr_nix_cache_<distro>)
- Prevent incompatible profile manifest versions between Ubuntu and Debian
- Update all test scripts (unit, integration, container, e2e)
- Remove unused global Nix volume variables from Makefile
- Improve consistency of test-e2e.sh formatting and environment handling
- Add Git safe.directory configuration for mounted /src to avoid ownership warnings

scripts/test/test-integration.sh

@@ -7,8 +7,8 @@ echo "============================================================"
 
 docker run --rm \
   -v "$(pwd):/src" \
-  -v pkgmgr_nix_store:/nix \
-  -v "pkgmgr_nix_cache:/root/.cache/nix" \
+  -v pkgmgr_nix_store_${distro}:/nix \
+  -v "pkgmgr_nix_cache_${distro}:/root/.cache/nix" \
   --workdir /src \
   -e PKGMGR_DEV=1 \
   -e TEST_PATTERN="${TEST_PATTERN}" \