Retry extension Cargo dependency fetch

The extension workflows build Pyrefly before packaging or testing the VS Code extension. A branch run hit a transient crates.io connection reset inside the actions-rust-cross cargo build step while fetching serde, which bypassed the retry wrapper because the fetch happened inside the third-party build action.

Fetch the Pyrefly Cargo dependencies explicitly through the local retry-command action before either the cross-compile action or native cargo build runs. For cross targets, include the matrix target in cargo fetch so target-specific dependencies are covered. This keeps the existing Rust cache useful on warm runs and gives cold or invalidated caches retry coverage before the expensive build step.