fix(ci): eliminate SHA propagation flakiness in Core Contrib Test (#4305)

* fix(ci): eliminate SHA propagation flakiness in Core Contrib Test

Each Core Contrib Test job was doing two separate git operations for
the core repo: actions/checkout (uses GitHub internal API, always
works) and then tox installing packages via git+https:// (hits public
CDN, fails when SHA hasn't propagated yet).

Fix by setting per-package env vars (CORE_REPO_API, CORE_REPO_SDK,
CORE_REPO_SEMCONV, CORE_REPO_TEST_UTILS) in the CI workflow to point
at the already-checked-out local copy, eliminating the second network
clone entirely.

Backward compatible: when the new vars are unset (local dev), they
fall back to the existing git URL + subdirectory behavior.

Fixes #4304

Assisted-by: Claude Sonnet 4.6

* fix: restore version header dropped during rebase

Assisted-by: Claude Sonnet 4.6

* move CORE_REPO_* vars to top-level env block

Use github.workspace context instead of a shell step to set the
per-package local path env vars, as suggested in code review.

Assisted-by: Claude Sonnet 4.6