ghc: fix parallel make

See comments at 93fe0378e7fbecf6ac9d0890cac2c1714c613a97

The changes I introduced at #21960 fixed parallel builds on the bots and on my machines. @mikemcquaid says he had problems with it on his machine although without additional details I can't tell if that was with the original patch or after switching back from ```GHC 7.6.3``` to ```GHC 7.4.2``` for bootstrapping.

Either way, using ```ENV.j1``` for installing the bootstrap binary is a bug because it ends up causing the entire formula to be built with a single job. I gather this is not what was intended by the fact that the secondary ```ENV.j1``` was left intact.

This patch fixes that issue by passing ```-j1``` explicitly instead of modifying the environment.

---

Test results from my 4-core ivy-bridge:

#### prior-change ####

```
real 80m36.601s
user 70m47.506s
sys  18m45.862s
```

#### post-change ####

```
real  29m53.582s
user 106m40.362s
sys   26m13.388s
```