Fix exit call in `SpackError.die()`
Fix exit call in `SpackError.die()`

- Previously we would use `os._exit()` in to avoid Spack error handling
  in the parent process when build processes failed.  This isn't
  necessary anymore since build processes propagate their exceptions to
  the parent process.

- Use `sys.exit` instead of `os._exit`. This has the advantage of
  automatically flushing output streams on quit, so output from child
  processes is not lost when Spack exits.

krafczyk/spack,lgarren/spack,krafczyk/spack,tmerrick1/spack,LLNL/spack,krafczyk/spack,mfherbst/spack,EmreAtes/spack,skosukhin/spack,TheTimmy/spack,tmerrick1/spack,TheTimmy/spack,matthiasdiener/spack,matthiasdiener/spack,LLNL/spack,mfherbst/spack,EmreAtes/spack,skosukhin/spack,lgarren/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,lgarren/spack,tmerrick1/spack,TheTimmy/spack,EmreAtes/spack,skosukhin/spack,tmerrick1/spack,iulian787/spack,matthiasdiener/spack,TheTimmy/spack,lgarren/spack,matthiasdiener/spack,matthiasdiener/spack,tmerrick1/spack,lgarren/spack,mfherbst/spack,iulian787/spack,skosukhin/spack,krafczyk/spack,EmreAtes/spack,EmreAtes/spack,LLNL/spack,mfherbst/spack,krafczyk/spack,skosukhin/spack,mfherbst/spack,TheTimmy/spack