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.

This changes the default parallelism for parallel compilers to one,
instead of the previous default, which was "num cpus". This is likely
not an optimal default long-term, but it is a good default for testing
whether parallel compilers are not a significant regression over a
sequential compiler.

Notably, this in theory makes a parallel-enabled compiler behave
exactly like a sequential compiler with respect to the jobserver.