Use mithril-client from PATH when available, download as fallback

The E2E test binary downloads mithril-client from GitHub releases at runtime (`Cardano.Launcher.Mithril.downloadMithril`), even though `mithril-client` is already available in the Nix shell via the flake input. Should check PATH first (`findExecutable "mithril-client"`), and only fall back to downloading when not found (Windows, non-Nix environments). This avoids version drift between the flake pin and the hardcoded download URL, and removes a network dependency from the test startup.