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
• tolerate runner images that do not provide /etc/apt/apt-mirrors.txt
• 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
• run the Windows PSD Welch regression test with cmd so %GITHUB_WORKSPACE% expands correctly

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 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. The file guard keeps the workflow compatible if a future hosted image changes its APT layout.

The first PR run also exposed a separate deterministic Windows failure: the PSD Welch test used cmd-style environment expansion under the default PowerShell shell. Explicitly selecting cmd makes it consistent with the adjacent Windows tests.

Validation

• CppCheck completed successfully using archive.ubuntu.com
• Valgrind completed successfully, including all build and test steps
• parsed all three modified workflow files as YAML
• git diff --check
• verified the mirror rewrite removes Azure while preserving the official fallback entries
• verified the guarded rewrite succeeds both with and without the mirror-list file
• all 11 GitHub Actions workflows passed on commit 233fe60