Only include space in RUSTFLAGS extra flags if not empty

When the RUSTFLAGS_STAGE_{1,2} is not set, including a space means
the string will always be non-empty and RUSTFLAGS will be always be
reset which breaks other ways of setting these such as through config
in CARGO_HOME.

src/bootstrap/builder.rs

@@ -555,7 +555,9 @@ impl<'a> Builder<'a> {
         let mut extra_args = env::var(&format!("RUSTFLAGS_STAGE_{}", stage)).unwrap_or_default();
         if stage != 0 {
             let s = env::var("RUSTFLAGS_STAGE_NOT_0").unwrap_or_default();
+            if !extra_args.is_empty() {
                 extra_args.push_str(" ");
+            }
             extra_args.push_str(&s);
         }