Ruby

Fix imemo_env corruption under auto compaction

Previously, vm_make_env_each() did:

1. ALLOC env_body
2. Copy locals into env_body
3. Allocate imemo_env
4. Set up imemo_env with env_body

If compaction runs during (3), locals copied to env_body could be
moved and the imemo_env could end up with invalid references.

Move (2) down so it reads references after potential movement.