snap-installed pdftk only allows for writing in home dir

[kevinchugh]

Would you please review this issue and make this change (I tried to do a pull request but the repo is closed)

in lib/pdf_forms/pdftk_wrapper.rb change line 43 from

tmp = Tempfile.new('pdf_forms-fdf')

to

tmp = Tempfile.new('pdf_forms-fdf', ENV["HOME"] or "/tmp")

This fixed our production pdf fillables on ubuntu 20.