ci: skip cache update on PRs. (#244)
ci: skip cache update on PRs. (#244)

Cache updates on PRs result in early eviction of primary cache,
due to the limited cache quota on GitHub Actions.

While there, remove fallback to restore cache key based only on
the Wasm runtime name, since it prevents pruning of stale data.

Signed-off-by: Piotr Sikora <ba6f3a1a6d7e1ecae91b31e93969cc392fbd0fdd@google.com>
proxy-wasm/proxy-wasm-cpp-host,proxy-wasm/proxy-wasm-cpp-host,proxy-wasm/proxy-wasm-cpp-host