Fix failing lints on source files

[NotTheDr01ds]

Since #791 (and possibly before), any non-module Nushell file pushed to nu_scripts will fail CI since it attempts to use <file.nu> during linting.

In addition, the #791 implementation could execute code during linting, since it actually imported potentially working code.

This PR updates the toolkit testing to:

• Determine whether to lint as a module or source file based on the presence of any export line in the file.
• Run nu-check on files before linting with use <file> or source <file>
• Updates the environment variable to TEST_METHOD with options for ide-check or import-or-source.
• Updates the default to import-or-source (was ide-check) to match CI
• Removes environment variable from CI since this test method is now the default.

With this in place we should have far fewer (false positive) failing CI runs.

[fdncred]

I'm fine with running with this to see how it works.