[analyzer] SATestBuild.py: Move additional checkers logic so SATestAdd.py can use it as well.

Move the logic looking for additional checkers in the SA_ADDITIONAL_CHECKERS
environmental variable from SATestBuild's main() to runScanBuild(). This allows
SATestAdd.py to use the variable as well. Without it, we won't include
additional checkers when building reference results for the build bot.

Differential Revision: http://reviews.llvm.org/D12891

llvm-svn: 247767