Harden Linux analysis workflows against APT mirror stalls

[Andrey1994]

Summary

• keep the Linux analysis workflows on the current stable ubuntu-latest image
• remove the unreliable first-priority Azure Ubuntu mirror before refreshing APT metadata
• bound the CppCheck and Valgrind jobs and their package-install steps
• consolidate Valgrind's three APT setup passes into one
• update actions/checkout and actions/setup-python to v7

Motivation

The CppCheck and Valgrind checks for #847 both stalled in apt-get update against azure.archive.ubuntu.com and were cancelled at GitHub's six-hour hosted-job limit:

• https://github.com/brainflow-dev/brainflow/actions/runs/30890111327
• https://github.com/brainflow-dev/brainflow/actions/runs/30890111514

Both runs already used Ubuntu 24.04.4, image 20260810.271.1. Identical workflows subsequently passed on the same image for other PRs, so changing or downgrading the runner image would not address the root cause.

GitHub's runner configuration already provides archive.ubuntu.com and security.ubuntu.com as fallback mirrors. Removing only the flaky Azure entry keeps those official fallbacks available while preventing another six-hour stall.

Validation

• parsed both modified workflow files as YAML
• git diff --check
• verified the mirror rewrite removes the Azure entry while preserving the official fallback entries