[linter] Fix shebang lint false positive

Summary:
Rust files can start with #![attr], but this is caught by lint-files.py by accident.

Instead, we check for #![^[], which should be more robust

Test Plan: arc lint --everything

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16980