speed up tests by doing reflink copies

pkgs/testing/flake-module.nix

@@ -15,8 +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 ${pkgs.coreutils}/bin/cp --recursive --target "$CLAN_TEST_STORE/nix/store"  < "$closureInfo/store-paths"
+            ${pkgs.findutils}/bin/xargs ${pkgs.xcp}/bin/xcp --recursive --target-directory "$CLAN_TEST_STORE/nix/store"  < "$closureInfo/store-paths"
             ${pkgs.nix}/bin/nix-store --load-db --store "$CLAN_TEST_STORE" < "$closureInfo/registration"
           fi
         '';
@@ -39,7 +38,7 @@
           ];
           postPatch = ''
             substituteInPlace nixos_test_lib/nix_setup.py \
-              --replace '@cp@' '${pkgs.coreutils}/bin/cp' \
+              --replace '@xcp@' '${pkgs.xcp}/bin/xcp' \
               --replace '@nix-store@' '${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
-CP_BIN = "@cp@"
+XCP_BIN = "@xcp@"
 NIX_STORE_BIN = "@nix-store@"
 XARGS_BIN = "@xargs@"
 
@@ -52,7 +52,7 @@ def setup_nix_in_nix(closure_info: str | None) -> None:
                 subprocess.run(  # noqa: S603
                     [
                         XARGS_BIN,
-                        CP_BIN,
+                        XCP_BIN,
                         "--recursive",
                         "--target-directory",
                         f"{tmpdir}/store/nix/store",