installcommand: use os.Exec unless told not to by INSTALLCOMMAND_NOEXEC
installcommand: use os.Exec unless told not to by INSTALLCOMMAND_NOEXEC

Because of uncertainties about os.Exec, we biased towards not exec'ing
unless told otherwise. With this change, we bias towards having
installcommand use os.Exec unless INSTALLCOMMAND_NOEXEC is set.
This leads to a much tidier process tree.

Signed-off-by: Ronald G. Minnich <f569d0af9a3481c4007b95c7180d6c41009f2513@gmail.com>

rminnich/u-root,u-root/u-root,hugelgupf/u-root,GanShun/u-root,u-root/u-root,hugelgupf/u-root,GanShun/u-root,hugelgupf/u-root,rminnich/u-root,rminnich/u-root,u-root/u-root,rminnich/u-root,GanShun/u-root,hugelgupf/u-root