go/runtime/host: Fix env vars for tests

[mitjat]

The test in sandbox_test.go only runs if OASIS_TEST_RUNTIME_HOST_BUNDLE_PATH is defined. However, we never set that in CI or elsewhere in the repo; we only set the older OASIS_TEST_RUNTIME_HOST_RUNTIME_PATH in CI here). That's probably a TODO from the migration to bundles.

The test running script (test_and_coverage.sh) needs to be updated to generate a bundle (oasis-node debug bundle) and set the right env variable.