Auto merge of #17975 - IvarWithoutBones:dont-assume-rustup, r=Veykril

fix: do not assume rustup is installed in xtask codegen

When formatting generated code the xtask crate attempts to run `rustup run stable rustfmt`, which fails if `rustup` is not installed. This results in test failures when another source manages the compiler toolchain, for example when using Nix (or any other distro-specific packaging solution):
* xtask::codegen::grammar::test
* xtask::codegen::assists_doc_tests::test

With this PR xtask will first attempt to run `rustup run stable rustfmt`, and if that fails just plain `rustfmt`. It still validates a stable version is being used. This allows `cargo test` to pass on systems that do not use `rustup`.

Author: bors