nix_setup/cp: remove xcp again

pkgs/testing/flake-module.nix

@@ -15,7 +15,7 @@
           mkdir -p "$CLAN_TEST_STORE/nix/store"
           mkdir -p "$CLAN_TEST_STORE/nix/var/nix/gcroots"
           if [[ -n "''${closureInfo-}" ]]; then
-            ${pkgs.findutils}/bin/xargs ${pkgs.xcp}/bin/xcp --recursive --target-directory "$CLAN_TEST_STORE/nix/store"  < "$closureInfo/store-paths"
+            ${pkgs.findutils}/bin/xargs -P"$(nproc)" ${pkgs.coreutils}/bin/cp --recursive --reflink=auto --target-directory "$CLAN_TEST_STORE/nix/store"  < "$closureInfo/store-paths"
             ${pkgs.nix}/bin/nix-store --load-db --store "$CLAN_TEST_STORE" < "$closureInfo/registration"
           fi
         '';
@@ -38,7 +38,7 @@
           ];
           postPatch = ''
             substituteInPlace nixos_test_lib/nix_setup.py \
-              --replace '@xcp@' '${pkgs.xcp}/bin/xcp' \
+              --replace '@cp@' '${pkgs.coreutils}/bin/cp' \
               --replace '@nix-store@' '${pkgs.nix}/bin/nix-store' \
               --replace '@xargs@' '${pkgs.findutils}/bin/xargs'
           '';

pkgs/testing/nixos_test_lib/nix_setup.py

@@ -5,7 +5,7 @@ import subprocess
 from pathlib import Path
 
 # These paths will be substituted during package build
-XCP_BIN = "@xcp@"
+CP_BIN = "@cp@"
 NIX_STORE_BIN = "@nix-store@"
 XARGS_BIN = "@xargs@"
 
@@ -45,15 +45,18 @@ def setup_nix_in_nix(closure_info: str | None) -> None:
     if closure_info and Path(closure_info).exists():
         store_paths_file = Path(closure_info) / "store-paths"
         if store_paths_file.exists():
-            # Use xargs to handle potentially long lists of store paths
-            # Equivalent to: xargs cp --recursive --target-directory
-            # "$CLAN_TEST_STORE/nix/store" < "$closureInfo/store-paths"
+            # Use xargs with parallel processing to copy store paths efficiently
+            # --reflink=auto enables copy-on-write when filesystem supports it
+            # -P uses all available CPU cores for parallel copying
+            num_cpus = str(os.cpu_count() or 1)
             with store_paths_file.open() as f:
                 subprocess.run(  # noqa: S603
                     [
                         XARGS_BIN,
-                        XCP_BIN,
+                        f"-P{num_cpus}",  # Use all available CPUs
+                        CP_BIN,
                         "--recursive",
+                        "--reflink=auto",  # Use copy-on-write if available
                         "--target-directory",
                         f"{tmpdir}/store/nix/store",
                     ],