flaky actions: skip actions/setup-python on musl (fixes Alpine flow tests)

[kei-nan]

Problem

flaky-filter and flaky-record-results run actions/setup-python@v6 unconditionally. actions/setup-python only publishes glibc CPython builds, so on musl hosts (e.g. Alpine) the interpreter it installs can't run — and worse, it exports LD_LIBRARY_PATH / pythonLocation into $GITHUB_ENV, which then corrupts the musl python the caller's job actually uses (RediSearch runs its flow tests against a uv musl venv).

Because these are composite actions, that env pollution persists to every later step in the caller's job. In RediSearch's test-*platforms / alpine:3.23 jobs (periodic + nightly validation, both x86_64 and aarch64) it produced:

• python3 -m pip install redis → /.../.venv/bin/python3: No module named pip
• the subsequent flow-test step → deps/readies/mk/main:49: *** Cannot find python3 interpreter while building RedisJSON → make pytest exit 2 → the job's "Fail flow if tests failed" gate goes red.

Every glibc platform (Ubuntu, macOS, Intel, coverage, sanitize) passed; only the two Alpine jobs failed, and identically.

Fix

Detect musl (/etc/alpine-release, or ldd --version reporting musl) and skip setup-python on musl, falling back to the ambient python3 the caller already provides (on RediSearch that's the musl uv venv, which has pip + redis). glibc/macOS behaviour is unchanged — setup-python still runs there.

Scope

Audited all 7 setup-python usages in this repo. Only these two composite actions run inside the caller's job (hence can land in a musl container). ci.yml (ubuntu-latest) and the flaky-mark/flaky-unmark/link-check/spellcheck reusable workflows (own jobs, ubuntu-slim, no container:) run on glibc and are unaffected.

Rollout

Consumers pin @v1 (a moving tag). After merge, advance v1 (and cut v1.0.2) to propagate.

🤖 Generated with Claude Code

---

Note

Low Risk
CI-only change limited to two composite actions; glibc paths still use setup-python, with musl relying on caller-provided python3.

Overview
Fixes Alpine container jobs where flaky-filter and flaky-record-results always ran actions/setup-python, which only ships glibc CPython and writes LD_LIBRARY_PATH / pythonLocation into $GITHUB_ENV, breaking the caller’s musl python3 (e.g. a uv venv).

Both composites now run detect_libc.sh after checking out redisearch-ci-common, set steps.libc.outputs.musl, and gate setup-python with if: steps.libc.outputs.musl != 'true' so musl hosts use ambient python3 for pip install redis and flaky_db.py. glibc/macOS behavior is unchanged.

The new scripts/ci_common/detect_libc.sh treats Alpine (/etc/alpine-release) or ldd --version reporting musl as musl=true and writes that to $GITHUB_OUTPUT.

^{Reviewed by Cursor Bugbot for commit 2a52cc4. Bugbot is set up for automated code reviews on this repo. Configure here.}