makebuilder: Change default number of jobs from 2 to 1

Default NumberOfJobs in makejob.cpp (1) and makebuilderconfig.kcfg (2)
is different. Kdevelop already tries to find the optimal number of threads
if NumberOfJobs field is not overridden. The current default makes
it impossible to override a number of jobs to 2 as NumberOfJobs will
switch to 1 due to makejob.cpp.

BUG: 448251
FIXED-IN: 5.8.220400