Function tvix_glue::tvix_build::handle_pass_as_file

fn handle_pass_as_file(
    environment_vars: &mut BTreeMap<String, Bytes>,
    additional_files: &mut BTreeMap<String, Bytes>
) -> Result<()>

handle passAsFile, if set. For each env $x in that list, the original env is removed, and a $xPath environment var added instead, referring to a path inside the build with the contents from the original env var.