fix: build PATH with platform separators

[dumanoglu1]

What changed

Config::execute_command now prepends entries to PATH with std::env::split_paths and std::env::join_paths instead of formatting a hard-coded : separator.

A small unit test covers that the helper preserves platform-specific path separators when prepending an entry.

Why

Windows uses ; as the PATH separator. The previous string formatting created malformed PATH values on Windows when adding the active toolchain opt/ directory or an overridden CARGO_HOME/bin.

Fixes #227

Validation

• cargo check
• cargo test prepending_a_path_entry_preserves_platform_separators was attempted, but the repository's current Windows test build fails first in unrelated test modules that call std::os::unix::fs::symlink from src/migrate_networks.rs and src/networks.rs.