Fix inconsistent shebang defaults

[hwikle-lanl]

Fixed default shebang value for ScriptComposer to be consistent with the test config default.

Code review checklist:

• Code is generally sensical and well commented
• Variable/function names all telegraph their purpose and contents
• Functions/classes have useful doc strings
• Function arguments are typed
• Added/modified unit tests to cover changes.
• New features have documentation added to the docs.
• New features and backwards compatibility breaks are noted in the RELEASE.md

[hwikle-lanl]

@pflarr We may want to change to defaulting to #!/bin/bash, but that would be a breaking change.