Set RUSTC and RUSTDOC env for child processes run through the proxy

This serves two purposes:
1. It avoids the overhead of having to re-parse `settings.toml` on each proxy.
   In projects with many crates, this can have non-trivial overhead.
2. For build scripts, it provides an absolute path for RUSTC rather than forcing them to look it up in PATH.
   This allows them to find the sysroot path for the toolchain, rather than ending up somewhere in CARGO_HOME.

The original motivation for this was to allow `bootstrap` in rust-lang/rust
to allow building stage0 tools without python, only the host rust toolchain.

Author: jyn514

src/toolchain.rs

@@ -401,6 +401,19 @@ impl<'a> InstalledCommonToolchain<'a> {
 
         cmd.env("RUSTUP_TOOLCHAIN", &self.0.name);
         cmd.env("RUSTUP_HOME", &self.0.cfg.rustup_dir);
+        // It's valid (although slightly cursed) to tell a rustup cargo proxy to use a custom rustc
+        // from a different toolchain.
+        if env::var("RUSTC").is_err() {
+            // Set RUSTC for the spawned binary. This serves two purposes:
+            // 1. It avoids the overhead of having to re-parse `settings.toml` on each proxy.
+            //    In projects with many crates, this can have non-trivial overhead.
+            // 2. For build scripts, it provides an absolute path for RUSTC rather than forcing them to look it up in PATH.
+            //    This allows them to find the sysroot path for the toolchain, rather than ending up somewhere in CARGO_HOME.
+            cmd.env("RUSTC", self.0.path.join("bin").join("rustc"));
+        }
+        if env::var("RUSTDOC").is_err() {
+            cmd.env("RUSTDOC", self.0.path.join("bin").join("rustdoc"));
+        }
     }
 
     fn set_ldpath(&self, cmd: &mut Command) {