ci: cache wasm blob between job runs

Our Go code (with 8582cb20a37d574e6397f980f919c6dd4dfb24a0) uses runtimes cached in the /tmp directory if available, and otherwise downloads them.

Sometimes Github can be a bit flaky serving those files over http, so the CI fails randomly because of it.

It might make sense to cache them between CI jobs/runners so the Go code doesn't have to fetch them.