fix(linters): pass build_base to linters and avoid hardcoded fallback…

[DRAKKKkk]

Fixes #6242

Description

Currently, linters only have access to the base key and not the build-base. This causes an issue in GpuLinter, where "core24" is hardcoded as a fallback if the base is "bare" or missing. This results in incorrect help URLs being generated for snaps that specify a different build-base (e.g., "core26").

This PR:

1. Updates the Linter base class (base.py) and run_linters (linters.py) to accept and store the build_base parameter.
2. Updates GpuLinter (gpu_linter.py) to fall back to self._build_base before defaulting to "core24".
3. Adds a unit test in test_gpu_linter.py to verify that build-base is correctly respected when base is "bare" or undefined.

QA Steps / Local Testing

• All unit tests for linters (pytest tests/unit/linters/) have passed successfully (146 passed).
• Code formatting and style checks (ruff format, ruff check) passed cleanly.

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• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• In documents I changed, I added a meta description if one was missing.
• I've updated the relevant release notes.

[DRAKKKkk]

"Hi @bepri and @mr-cal, I've submitted a fix for #6242.

The PR propagates the build_base parameter through the Linter base class and updates GpuLinter to prioritize it over the hardcoded fallback. I've also added test coverage for the fallback resolution logic, and all 146 tests are passing cleanly.

I look forward to your review and feedback. Thanks!