Update build instructions

[Nothing4You]

build.sh doesn't exist (anymore).
I guess it should refer to make instead.

[Nothing4You]

I'm not a go dev but I don't really understand how this would currently build anyways:

$ make iso-macos
go mod download
go: github.com/hashicorp/packer@v1.4.2 requires
	github.com/gophercloud/gophercloud@v0.0.0-20180903124057-ea7289ebdf06: invalid pseudo-version: does not match version-control timestamp (2018-09-17T09:41:35Z)
make: *** [modules] Error 1

[Nothing4You]

Considering that gophercloud now has tagged releases it might also be a good idea to update to one of those. (it's a packer dep, not this plugin's dep)

[Nothing4You]

Changing the packer dep from 1.4.2 to 1.4.4 fixed the dependency errors, their dependency was updated 4 months ago and shipped in 1.4.3.