Mark install-packages +x.

install-packages is an executable script, it should be +x.  The install
command that was installing it is setting permissions to 755, so we were
fine there.  However, I was trying to test the script outside of an
image build to test a change, and it's helpful to have it +x in the
repository for that context like all the other scripts under bin.

Change-Id: Ic300cc56d463640d119db4e310d375c66ed133ca