Invoke local build only if rust toolchain available.
Invoke local build only if rust toolchain available.

Close #429.

autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim