qibuild.sh: add change_cwd
qibuild.sh: add change_cwd

to be used in a `with` statement:

  with change_cwd(my_dir):
      # do stuff in my_dir

  # now back in previous cwd

Change-Id: I2a8ece75aa5eac2d7b547b904efd031cb3334a7d

dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild