bootstrap: Move `-Zdual-proc-macros` to `builder.rs`

src/bootstrap/bin/rustc.rs

@@ -124,12 +124,6 @@ fn main() {
             cmd.arg("--sysroot").arg(&sysroot);
         }
 
-        // Link crates to the proc macro crate for the target, but use a host proc macro crate
-        // to actually run the macros
-        if env::var_os("RUST_DUAL_PROC_MACROS").is_some() {
-            cmd.arg("-Zdual-proc-macros");
-        }
-
         // When we build Rust dylibs they're all intended for intermediate
         // usage, so make sure we pass the -Cprefer-dynamic flag instead of
         // linking all deps statically into the dylib.

src/bootstrap/builder.rs

@@ -802,7 +802,7 @@ pub fn cargo(
                 // Build proc macros both for the host and the target
                 if target != compiler.host && cmd != "check" {
                     cargo.arg("-Zdual-proc-macros");
-                    cargo.env("RUST_DUAL_PROC_MACROS", "1");
+                    rustflags.arg("-Zdual-proc-macros");
                 }
             },
         }