fixup icepick workflow argument requirements

crates/icepick/src/cli/workflow.rs

@@ -88,8 +88,9 @@ pub fn generate_command(workflow: &Workflow) -> clap::Command {
             "A file containing any inputs not passed on the command line"
     ));
     for input in &workflow.inputs {
+        // can also be included in the JSON file, so we won't mark this as required.
         let arg = clap::Arg::new(input)
-            .required(true)
+            .required(false)
             .long(input.replace('_', "-"))
             .value_name(input.to_uppercase());
         command = command.arg(arg);