Arguments to fetchers are (at least sometimes) not parsed as the right type

[malob]

I just tried to use nix-universal-prefetch with fetchzip, and I needed to use the --stripRoot argument, but this errors:

❯ nix run my#nix-universal-prefetch -- fetchzip --stripRoot false --url https://cache.agilebits.com/dist/1P/op2/pkg/v2.0.0/op_linux_386_v2.0.0.zip
nix-universal-prefetch: Couldn't parse the error output...
error: value is a string while a Boolean was expected

       at /nix/store/2hk95qkrlxwcaphx9ryw6bhqphpaixfq-source/pkgs/build-support/fetchzip/default.nix:48:8:

           47|     ''
           48|     + (if stripRoot then ''
             |        ^
           49|       if [ $(ls "$unpackDir" | wc -l) != 1 ]; then