[gn] Fix --sysroot and --clang_prefix values

These calculations were conflating root_build_dir with root_out_dir.
They are sometimes the same but not always.

Bug: SEC-27
Test: Build with select_variant=["asan"]
Change-Id: Ie0bd37356de67b621888189a4b26ce3de03701f6
diff --git a/rustc_deps/BUILD.gn b/rustc_deps/BUILD.gn
index ff3630f..8e0b3f3 100644
--- a/rustc_deps/BUILD.gn
+++ b/rustc_deps/BUILD.gn
@@ -76,8 +76,8 @@
     "--cmake-dir", rebase_path("//buildtools/cmake/bin"),
     "--shared-libs-root", rebase_path(
       get_label_info("//default($shlib_toolchain)", "root_out_dir")),
-    "--clang_prefix", rebase_path(clang_prefix, "", root_out_dir),
-    "--sysroot", rebase_path("$root_out_dir/sdks/zircon_sysroot/sysroot"),
+    "--clang_prefix", rebase_path(clang_prefix, "", root_build_dir),
+    "--sysroot", rebase_path("$root_build_dir/sdks/zircon_sysroot/sysroot"),
   ]
 
   outputs = [ out_deps_data ]