c2rust-transpile: Use a separate file for the generated rust-toolchain.toml

When c2rust-transpile translates C variadics, it generates code using
the (currently) nightly-only feature `c_variadics`, so it emits a
`rust-toolchain.toml` file to use nightly. Rather than copy this file
from the `rust-toolchain.toml` used to build c2rust itself, use a
separate file.

This also means the generated `rust-toolchain.toml` can stop requiring
toolchain components that the generated code doesn't need, such as
rustc-dev. (It still requires `rustfmt`, as c2rust may run `cargo fmt`
on the generated code if invoking `c2rust-refactor`.)

Author: joshtriplett

c2rust-transpile/src/build_files/generated-rust-toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "nightly-2022-08-08"
+components = ["rustfmt"]

c2rust-transpile/src/build_files/mod.rs

@@ -260,7 +260,7 @@ fn emit_lib_rs(
 /// on a nightly toolchain until the `c_variadics` feature is stable.
 fn emit_rust_toolchain(tcfg: &TranspilerConfig, build_dir: &Path) {
     let output_path = build_dir.join("rust-toolchain.toml");
-    let output = include_str!("../../rust-toolchain.toml").to_string();
+    let output = include_str!("generated-rust-toolchain.toml").to_string();
     maybe_write_to_file(&output_path, output, tcfg.overwrite_existing);
 }