[dev_tools] Update args in local sdk script

The args - `cached_package_labels` is added in core gni. This CL
updates the args in local-sdk script to prevent local script failures.

Bug: None
Test: manual testing
Change-Id: I4d7ad136e461551540180ff463b1194a3bd42dcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252567
Auto-Submit: Arun Manickam <arunmanickam@google.com>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780259}

tools/fuchsia/local-sdk.py

@@ -37,7 +37,7 @@ def EnsureEmptyDir(path):
 def BuildForArch(arch):
   build_dir = 'out/release-' + arch
   Run('scripts/fx', '--dir', build_dir, 'set', 'core.qemu-' + arch,
-      '--args=cache_package_labels=["//sdk/bundles:tools"]',
+      '--args=cache_package_labels+=["//sdk/bundles:tools"]',
       '--args=is_debug=false', '--args=build_sdk_archives=true')
   Run('scripts/fx', 'build', 'sdk', 'build/images')