Remove absolute paths in ninja files for v8windbg gn file

No behavior change.

Bug: chromium:1278777
Change-Id: I18deed9571acb9f953cb6cddee12e27733de98b5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3332197
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78352}

tools/v8windbg/BUILD.gn

@@ -75,8 +75,8 @@ action("copy_prereqs") {
   outputs = [ "$root_out_dir/dbgeng.dll" ]
 
   args = [
-    rebase_path("//build"),
-    rebase_path(root_out_dir),
+    rebase_path("//build", root_build_dir),
+    rebase_path(root_out_dir, root_build_dir),
     target_cpu,
   ]
 }