[cts] Use build-relative paths for cache copy script

Instead of BUILD.gn relative paths.

'chromium/ci/Windows deterministic' doesn't like it if an action has the
build directory emitted into the toolchain.ninja file - it considers
that a diff.

Bug: chromium:1503147
Change-Id: I3c4679e2ad585bec3fa32a0f4fdc0b4ba2a9730b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/161800
Kokoro: Ben Clayton <bclayton@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Bruce Dawson <brucedawson@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/third_party/gn/webgpu-cts/BUILD.gn b/third_party/gn/webgpu-cts/BUILD.gn
index f9f71cb..e0a77a2 100644
--- a/third_party/gn/webgpu-cts/BUILD.gn
+++ b/third_party/gn/webgpu-cts/BUILD.gn
@@ -109,11 +109,11 @@
   script = "${dawn_root}/webgpu-cts/scripts/copy_files.py"
   args = [
     "--src_dir",
-    rebase_path(src_dir),
+    rebase_path(src_dir, root_build_dir),
     "--dst_dir",
-    rebase_path(dst_dir),
+    rebase_path(dst_dir, root_build_dir),
     "--file_list",
-    rebase_path("resource_files.txt"),
+    rebase_path("resource_files.txt", root_build_dir),
     "--stamp",
     rebase_path(outputs[0], root_build_dir),
   ]