Get build_deb_packages.sh working
Get build_deb_packages.sh working

* Update the path to the fpf-signing-key file, which was moved
  in a refactor.
* Always create the build artifacts directory to avoid an error on first
  run

harlo/securedrop,garrettr/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,jrosco/securedrop,heartsucker/securedrop,chadmiller/securedrop,chadmiller/securedrop,heartsucker/securedrop,harlo/securedrop,ageis/securedrop,GabeIsman/securedrop,chadmiller/securedrop,ageis/securedrop,kelcecil/securedrop,garrettr/securedrop,conorsch/securedrop,harlo/securedrop,harlo/securedrop,conorsch/securedrop,kelcecil/securedrop,jaseg/securedrop,jrosco/securedrop,chadmiller/securedrop,kelcecil/securedrop,GabeIsman/securedrop,jeann2013/securedrop,jrosco/securedrop,pwplus/securedrop,harlo/securedrop,jaseg/securedrop,jaseg/securedrop,chadmiller/securedrop,micahflee/securedrop,pwplus/securedrop,jrosco/securedrop,ehartsuyker/securedrop,garrettr/securedrop,jrosco/securedrop,heartsucker/securedrop,harlo/securedrop,pwplus/securedrop,jeann2013/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,GabeIsman/securedrop,kelcecil/securedrop,GabeIsman/securedrop,ehartsuyker/securedrop,micahflee/securedrop,jaseg/securedrop,conorsch/securedrop,kelcecil/securedrop,GabeIsman/securedrop,micahflee/securedrop,ehartsuyker/securedrop,conorsch/securedrop,pwplus/securedrop,pwplus/securedrop,GabeIsman/securedrop,ageis/securedrop,kelcecil/securedrop,garrettr/securedrop,jaseg/securedrop,chadmiller/securedrop,micahflee/securedrop,jaseg/securedrop,conorsch/securedrop,pwplus/securedrop,jeann2013/securedrop,ageis/securedrop,ehartsuyker/securedrop,jeann2013/securedrop,jeann2013/securedrop,jeann2013/securedrop,jrosco/securedrop