Test features against shokkunrf base image and on arm64

[shokkunrf]

Summary

• Cover untested setups: add shokkunrf-base scenario to all five feature tests (verifies install on the project's own base image) and devcontainer-base-debian-with-python to gcloud (verifies the common combination with the python feature)
• Catch ARM-specific regressions hitting Apple Silicon users: run feature CI on both amd64 (ubuntu-latest) and arm64 (ubuntu-24.04-arm) via a feature × arch matrix
• Make test-only edits actually trigger CI: extend the detect job to inspect features/test/ diffs (previously only features/src/ was tracked, so PRs touching only scenarios never fired the matrix)
• Fix gcloud install on ARM and on minimal images: Google's x86_64 tarball ships a bundled Python while the ARM tarball does not, and some minimal images ship python3 without the sqlite3 stdlib module. Ensure a system python3 with sqlite3 (installs via apt-get when missing), drop the bundled Python so behavior is uniform across architectures, and export CLOUDSDK_PYTHON during the SDK's install. README clarifies the apt-get requirement and Alpine non-support