Add Linux arm64 CI, pin GitHub Actions to hashes

[agriyakhetarpal]

Description

• Linux arm64 runners are now generally available for GitHub Actions usage.
• Pins all GitHub Actions used to their hashes instead of using their versions.

Additional context

• https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/
• https://docs.github.com/en/actions/security-for-github-actions/security-guides/security-hardening-for-github-actions

[agriyakhetarpal]

xref actions/setup-python#1010 and actions/setup-python#1011 for enabling PyPy on Linux arm64. I can do that when there's a new release. Merging this as all tests pass, as expected – nightly failures are tracked in #671 already.