Update brave license helper for new rust vendor location

[rillian]

Description

In brave/brave-core#21759 we move third_party rust source directories to a separate subtree away from their documenting README.chromium files. This aligns with what chromium's tooling currently expects, but confuses the license aggregation script. This bug is about updating brave_license_header.py to handle the new locations, or some other appropriate change.

The gnrt tool does check for compatible license declarations in Cargo.toml when importing source, and embeds them in the README, but it often isn't smart enough to generate a path for the License File key.