Don't use stage naming in RUSTFLAGS environment variables

This patch supports less behavior than before, since specifiying stage 1
vs stage 2 is no longer possible, but that is presumably a somewhat rare
use case anyway, so not supporting it seems acceptable (and it can be
readded easily if desired).

src/bootstrap/builder.rs

@@ -859,12 +859,12 @@ impl<'a> Builder<'a> {
             stage = compiler.stage;
         }
 
-        let mut extra_args = env::var(&format!("RUSTFLAGS_STAGE_{}", stage)).unwrap_or_default();
+        let mut extra_args = String::new();
         if stage != 0 {
-            let s = env::var("RUSTFLAGS_STAGE_NOT_0").unwrap_or_default();
-            if !extra_args.is_empty() {
-                extra_args.push_str(" ");
-            }
+            let s = env::var("RUSTFLAGS_NOT_BOOTSTRAP").unwrap_or_default();
+            extra_args.push_str(&s);
+        } else {
+            let s = env::var("RUSTFLAGS_BOOTSTRAP").unwrap_or_default();
             extra_args.push_str(&s);
         }