ci: avoid deadsnakes in GPU workflows

[ivanbasov]

Summary

• remove the GPU workflows' dependency on ppa:deadsnakes/ppa
• install Python 3.11/3.12/3.13 with actions/setup-python@v5 instead
• keep apt setup limited to stable system packages (git, git-lfs, gcc)
• add the missing PR skip gate for multi-gpu-tests

Context

PR #75 and the test-only PR #76 both failed before checkout/tests in CI / GPU.
The failing step was Install system dependencies, where add-apt-repository -y ppa:deadsnakes/ppa
timed out while contacting Launchpad:

TimeoutError: [Errno 110] Connection timed out

Because PR #76 only changes README text and reproduced the same setup failure, this looks
independent of PR #75's code changes.

After the setup fix, multi-gpu-tests also became reachable on the PR path and queued on
the 2-GPU runner. Its comment says it is main-only, so this PR adds the missing
github.ref == 'refs/heads/main' gate.

Validation

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

actionlint is not installed in the local environment.