ci: avoid flaky GPU apt cache

[ivanbasov]

Summary

• remove the NVIDIA proxy-cache setup from GPU workflows
• use the normal Ubuntu apt sources for the small system package install

Context

PR #75 now uses the fixed Python setup from #78, but GPU jobs are failing before checkout/tests because the NVIDIA apt cache returns 502 Bad Gateway for the Ubuntu archive endpoints. The resulting incomplete apt index makes packages such as gcc, git, and Python dependencies appear unavailable.

The first attempt tried enable-apt: false, but the action still rewrote apt sources. Removing the action avoids making CI depend on the local apt cache health.

Validation

• Parsed .github/workflows/ci-gpu.yml and .github/workflows/long-running-tests.yml with PyYAML
• Ran git diff --check