Fix modification of package command string

As Nick pointed out in a3ef60bc75c33d3309fc3375b7485621d3b4a285,
the way that this code was implemented would have the undesirable
side effect of modifying the value of the provider's `@resource[:name]`
value.  Changing the code from `<<` to `+=` causes the string
to be copied rather than modifying via reference.