Clean up e2e test for linux

[ehmatthes]

On Linux, need to use shell=True, and whole commands instead of parts. There are a bunch of conditional tests for Linux, that can be simplified.

• Update make_sp_call() in core e2e test utils to accept use_parts arg.
• Simplify existing checks about Windows, heroku, etc. make_sp_call() should not do any of that inspection.
• Simplify calls in e2e test utils for this plugin with the updated core test utility function.
• See also core issue 405, about removing use of shell=True.

[ehmatthes]

Working notes

• Can use shutil.which("fly") to find path to flyctl executable. I believe this is how we detect poetry elsewhere in core.