[ddc] Update DDC snapshot training target

Specify package_config.json when running DDC JIT snapshot training.
This allows the build to succeed in environments where the running
compiler would go searching for the package_config.json and find
one in some parent directory. This should fix local web engine
builds in Flutter which likely broke when the repo was migrated to a
pub workspace.

Change-Id: Ifb5b6357cbf40f04069020ab1b4aee099d0945ed
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/438602
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

utils/ddc/BUILD.gn

@@ -40,6 +40,8 @@ application_snapshot("dartdevc") {
   main_dart = "../../pkg/dev_compiler/bin/dartdevc.dart"
 
   training_args = [
+    "--packages",
+    rebase_path("../../.dart_tool/package_config.json"),
     "--dart-sdk-summary",
     rebase_path(sdk_outline_dill),
     "-o",