Should not call rustfmt --help unconditionally

[joshtriplett]

• rust.vim version: Latest from git (4aa69b8)

Steps to reproduce:

• Create a new Cargo project.
• Create a file rust-toolchain containing:

[toolchain]
channel = "nightly-2022-06-30"
components = ["cargo", "llvm-tools-preview", "rust-src", "rust-std", "rustc", "rustc-dev", "rustfmt"]

• (This assumes you don't have that toolchain installed.)
• Edit the source file.
• ZZ

Expected vs. actual behavior:

I expected this to quickly save and exit. Instead, it hung with no visible output. I had to strace it to realize that it was running rustfmt --help, which was updating itself via rustup because of the rust-toolchain file.

Please don't run rustfmt --help unless actually running rustfmt.

Paste debugging info from the Rust Vim plugin via one of the following
commands: :RustInfo, :RustInfoToClipboard, or :RustInfoToFile <filename>.

rust.vim Global Variables:
let g:ftplugin_rust_source_path = v:null
let g:loaded_syntastic_rust_cargo_checker = v:null
let g:loaded_syntastic_rust_filetype = v:null
let g:loaded_syntastic_rust_rustc_checker = v:null
let g:rust_bang_comment_leader = v:null
let g:rust_cargo_avoid_whole_workspace = v:null
let g:rust_clip_command = v:null
let g:rust_conceal = v:null
let g:rust_conceal_mod_path = v:null
let g:rust_conceal_pub = v:null
let g:rust_fold = v:null
let g:rust_last_args = v:null
let b:rust_last_args = []
let g:rust_last_rustc_args = v:null
let b:rust_last_rustc_args = []
let g:rust_original_delimitMate_excluded_regions = v:null
let g:rust_playpen_url = v:null
let g:rust_prev_delimitMate_quotes = v:null
let g:rust_recent_nearest_cargo_tol = v:null
let g:rust_recent_root_cargo_toml = v:null
let g:rust_recommended_style = v:null
let g:rust_set_conceallevel = v:null
let g:rust_set_conceallevel=1 = v:null
let g:rust_set_foldmethod = v:null
let g:rust_set_foldmethod=1 = v:null
let g:rust_shortener_url = v:null
let g:rustc_makeprg_no_percent = v:null
let g:rustc_path = v:null
let g:rustfmt_autosave = 0
let g:rustfmt_autosave_if_config_present = v:null
let g:rustfmt_command = 'rustfmt'
let g:rustfmt_emit_files = 1
let g:rustfmt_fail_silently = 0
let g:rustfmt_options = ''
let g:syntastic_extra_filetypes = ['rust']
let g:syntastic_rust_cargo_fname = v:null
rustfmt 1.4.38-stable (a8314ef 2022-06-27)
rustc 1.62.0 (a8314ef7d 2022-06-27)
cargo 1.62.0 (a748cf5a3 2022-06-08)

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