`formatOnSave` with VSCode extension does not respect `ignore` config in rustfmt.toml

Ran into this while working in the rustc repo. There are a number of paths in rustfmt.toml that are ignored, but if you open and then save any of them, then the rust-analyzer VSCode extension still formats the contents with rustfmt.

Minimal steps to reproduce

1. Create empty project with cargo new.

2. Write some poorly formatted code to src/main.rs:

    fn   main ()
    {
   println!("Hello, world!");
    }
   

3. Ignore src/main.rs in rustfmt.toml:

   ignore = [
       "src/main.rs",
   ]
   

4. Demonstrate that nightly rustfmt respects this configuration:

   $ cargo +nightly fmt -- src/main.rs
   $ cat src/main.rs
    fn   main ()
    {
   println!("Hello, world!");
    }
   

5. Enable formatOnSave using nightly rustfmt in one of two ways:

   Option 1: rust-toolchain

   In rust-toolchain:

   nightly
   

   In .vscode/settings.json:

   {
       "editor.formatOnSave": true
   }
   

   Option 2: overrideCommand

   In .vscode/settings.json:

   {
       "rust-analyzer.rustfmt.overrideCommand": [
           "/YOUR/HOME/.rustup/toolchains/nightly-YOUR-HOST-TRIPLE/bin/rustfmt"
       ],
       "editor.formatOnSave": true
   }
   

6. Open src/main.rs in VSCode and save.

Observed Behavior

The rust-analyzer extension formats the code in src/main.rs.

Expected behavior

The rust-analyzer extension respects the configuration in rustfmt.toml and
does not format the code in src/main.rs.

Start with the rust-analyzer VSCode extension's formatOnSave path and compare its rustfmt invocation with cargo +nightly fmt using the rustfmt.toml ignore setting. Reproduce with .vscode/settings.json and the ignored src/main.rs example. Done means saving the ignored file leaves it unchanged, with coverage for the configuration and format-on-save behavior.