Revert "rustc: don't build on i686"

This reverts commit 9f86136cef.

Rust is nowadays required for building Firefox, so the channel updates
are blocked on this.

(It also builds fine for me.)

pkgs/development/compilers/rust/rustc.nix

@@ -153,6 +153,6 @@ stdenv.mkDerivation {
     description = "A safe, concurrent, practical language";
     maintainers = with maintainers; [ madjar cstrahan wizeman globin havvy wkennington retrry ];
     license = [ licenses.mit licenses.asl20 ];
-    platforms = subtractLists platforms.i686 (platforms.linux ++ platforms.darwin);
+    platforms = platforms.linux ++ platforms.darwin;
   };
 }