--downloader_config should support %workspace% expansion

### Description of the feature request:

Should make it more clear for (workspace-)relative paths that they are picked up from workspace. Easier to see dependencies when transferring or reusing .bazelrc between workspaces

### Which category does this issue belong to?

_No response_

### What underlying problem are you trying to solve with this feature?

_No response_

### Which operating system are you running Bazel on?

_No response_

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start by locating Bazel's --downloader_config handling and .bazelrc parsing; the issue names no files or tests. Clarify the expected workspace-relative expansion behavior, then verify that %workspace% resolves from the workspace without changing existing behavior.

Written by the indexing model from the issue text.