ci: add retries to e2e install/setup steps to reduce canary flakiness

[bobbor]

Description of changes

The daily on-schedule-canary-test.yml canaries have been failing intermittently (4 of the last 15 runs, each a different suite). All but one failure were transient network errors during dependency install/setup steps — the retry_count: 3 input only wraps test execution, while install/setup steps had no retry:

Run	Failed suite	Cause
2026-07-13	integ_react_storage	plain run: yarn — better-sqlite3 prebuild download timeout, then getaddrinfo EAI_AGAIN nodejs.org during node-gyp fallback
2026-07-09	integ_react_graphql_api, integ_react_datastore_selective_sync	Failed to download binary (statusCode 504): Gateway Time-out
2026-07-02	integ_rollup_datastore_basic_crud	git failure (exit code 128) during samples-staging setup

Changes:

• .github/workflows/callable-canary-e2e-tests.yml: the Install step now uses the repo's existing scripts/retry-yarn-script.sh -s install -n 3 (same pattern as callable-e2e-test-detox.yml) instead of a bare yarn.
• .github/actions/setup-samples-staging/action.yml: the staging-branch selection is wrapped in a 3-attempt retry loop that distinguishes a genuinely missing branch (git ls-remote --exit-code → 2, falls back to default branch, no retry) from transient git/network failures (e.g. exit 128, retried). Emits a ::warning:: annotation if retries are exhausted.

The test-execution retry logic consuming retry_count is unchanged.

Validation

• Both YAML files parse cleanly (yaml.safe_load).
• The new branch-selection logic was functionally tested under bash -e -o pipefail with a mocked git covering: branch exists (checkout), branch missing (rc=2 → default branch), transient rc=128 (retries then falls back), fetch failure after successful ls-remote (retries).
• CI-only change — no changeset needed.

Out of scope

• The 2026-06-30 failure (integ_react_datastore_multi_auth_three_plus_rules, Cypress assertion 'Current User: test01' never rendered) is a genuine test-level flake not addressed by install retries.
• callable-canary-sampleapp-tests.yml still has unretried npx create-react-app/npm install steps — candidate for a follow-up.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 95e1b59

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR