test(bigframes): Disable ibis fallback for system tests

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces a new session-scoped, autouse pytest fixture use_sqlglot_compiler in packages/bigframes/tests/system/conftest.py. This fixture configures the SQL compiler option to "experimental" for the duration of the test session and restores the original setting afterward. There are no review comments, and I have no feedback to provide.

[tswast]

Thanks!