Unify paths for custom runtime packs

[davidnguyen-tech]

Fixes #24339 according to the proposed solution.

In summary:

• The updated Makefile was the only place where we looked for custom runtime packs under Debug
  • This was introduced recently in f4fb0e8
• The rest of the code base (e.g. build-custom-runtime.sh) uses Release
• This PR unifies the paths to use Release, as it was before.

[rolfbjarne]

Re-created this PR as #24342 because our CI currently doesn't work with PRs from forks.