rustup: if the user only has one toolchain, use it (#340)
rustup: if the user only has one toolchain, use it (#340)

This fixes one common use case of #339, but not all. If the user only has one
toolchain, and it's a nightly, just use that instead of doing the
configuration prompt dance.
KalitaAlexey/vscode-rust,KalitaAlexey/vscode-rust,editor-rs/vscode-rust,editor-rs/vscode-rust