--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?

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

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Which operating system are you running Bazel on?

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What is the output of bazel info release?

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

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

Have you found anything relevant by searching the web?

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

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