use RUSTC environment var in parse_and_replace::compile if present

Cargo respects this environment variable (to specify the path to what
rustc binary to use), but the Rustfix test suite did not. However,
this capability is useful when developing new compiler diagnostics
that one wants Rustfix to be able to handle (this being inspired by
work on the endeavor that is rust-lang/rust#53013).

tests/parse_and_replace.rs

@@ -13,6 +13,7 @@ extern crate difference;
 
 use std::collections::HashSet;
 use std::ffi::OsString;
+use std::env;
 use std::fs;
 use std::path::{Path, PathBuf};
 use std::process::Output;
@@ -51,7 +52,7 @@ fn compile(file: &Path, mode: &str) -> Result<Output, Error> {
         args.push("--edition=2018".into());
     }
 
-    let res = duct::cmd("rustc", &args)
+    let res = duct::cmd(env::var_os("RUSTC").unwrap_or("rustc".into()), &args)
         .env("CLIPPY_DISABLE_DOCS_LINKS", "true")
         .env_remove("RUST_LOG")
         .stdout_capture()