feat: add executable flag to pixi run

[kajal-jotwani]

Description

Adds a --executable (-x) flag to pixi run to allow running executables even when a task with the same name exists.

Fixes #5128

How Has This Been Tested?

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Checklist:

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[baszalmstra]

This is a bit of an anti-pattern. If we have multiple booleans as arguments it becomes hard to distinguish what the false indicates. Instead its good practice to introduce an enum. E.g. PreferExecutable with variants like Always, Never, IfNoTask or something along those lines.

[kajal-jotwani]

Thanks for the suggestion! I've updated the implementation to use a PreferExecutable enum with Never and Always variants instead of bool. I didn't include an IfNoTask variant since that behavior is already the default when using PreferExecutable::Never the code naturally falls back to executing the command when no matching task is found.