experimental_repository_disable_download fails with blzmod enabled

The required packages being available in local repositories, but --experimental_repository_disable_download will error out when bzlmod is enabled.

To reproduce the bug:

1. Clear the bazel cache - rm -rv $(bazelisk info repository_cache)

2. Build with downloads disabled - bazelisk build --experimental_repository_disable_download //:main

   This will fail, with the error "no such package '@@platforms//host': java.io.IOException: Failed to download repository @@platforms: download is disabled."

3. Disable blzmod and try again - bazelisk build --experimental_repository_disable_download --enable_bzlmod=false //:main

This will pass.