v1.0.15

fix: use cache-directories instead of workspaces for absolute target-dir

rust-cache's `workspaces` input uses `path.join(root, target)` which
on Windows concatenates the workspace root with an absolute path on a
different drive, producing invalid paths like `D:\a\repo\E:\target`.

Instead of passing target-dir via workspaces:
- Set `cache-targets: false` when target-dir is set (no ./target exists)
- Set `cache-directories` to the target dir (adds paths directly without path.join)
- Revert to upstream Swatinem/rust-cache (no fork needed)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>