Fix usage of -with-rtsopts (#429)

When I added `-qg` to the default RTS options in
57cfa03c (#385) I neglected to consider that it
the -with-rtsopts flag would override the previous flag setting `-I0`. This
had the effect of reenabling idle GC, causing GC time to regress terribly. I
likely didn't notice this since I had passed the flags directly to the `hadrian`
executable with `+RTS` while testing.

Moreover, I mistakenly wrote `-qg0`, which (somewhat confusingly)
actually *enables* parallel GC. Instead I wanted to write `-qg`.