mkosi: extend --environment to cover all scripts

mkosi: extend --environment to cover all scripts

I would like to set environment variables for other scripts. But I
don't think adding separate options make sense: it should be fine to
just set the same environment variable for all scripts that are
invoked… After all, variables are best for "global" settings. Scripts
already get positional arguments that allow them to distinguish build
phases, so they don't need to use variables for this.