Make all alt builders produce parallel-enabled compilers
Make all alt builders produce parallel-enabled compilers

We're not quite ready to ship parallel compilers by default, but the alt
builders are not used too much (in theory), so we believe that shipping
a possibly-broken compiler there is not too problematic.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust