[core] Prestart worker with runtime env

[dentiny]

Troubleshooting result for runtime env performance issue:

• if we clear runtime env before prestart (clear before prestart #49952), the perf is good
• if we clear it after (clear before queue and schedule #49951), the perf is bad

Reading through the code, if runtime env empty, we prestart worker, otherwise we skip:

ray/src/ray/raylet/worker_pool.cc

Line 1451 in 3bd3865

task_spec.HasRuntimeEnv() || task_spec.GetLanguage() != ray::Language::PYTHON) {

After the change, the performance is much better: https://buildkite.com/ray-project/release/builds/30878

[jjyao]

Add some tests

[dentiny]

Add some tests

Do you think it good enough to serve as assertion on #50003?

[jjyao]

LG. We can merge after adding tests.

[dentiny]

LG. We can merge after adding tests.

Test added