CI: github: cache QuicTLS builds

CI: github: cache QuicTLS builds

Resolve the QuicTLS ref to a commit SHA when generating the VTest
matrix and include it in the SSL build inputs and cache key. Keep job
labels readable by showing the ref and an eight-character SHA.

Fetch and check out the resolved commit in build-ssl.sh while retaining
QUICTLS_VERSION for the OpenSSL 1.1.1 build target selection. Preserve
the existing download behavior when QUICTLS_COMMIT is unset.

Enable SSL caching for QuicTLS.