[Bugfix] fix nightly multi-node EPLB tests' "DYNAMIC_EPLB=true" environment not working (#4223)

### What this PR does / why we need it?
fix nightly multi-node EPLB tests by adjusting
vllm_ascend\eplb\core\eplb_utils.py dynamic_eplb gate checking
### Does this PR introduce _any_ user-facing change?
no

- vLLM version: v0.11.0
- vLLM main:
2918c1b49c

---------

Signed-off-by: 白永斌 <baiyongbin3@h-partners.com>
Signed-off-by: 欧派果奶我还要 <47294568+845473182@users.noreply.github.com>
Co-authored-by: 白永斌 <baiyongbin3@h-partners.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

vllm_ascend/envs.py

@@ -172,7 +172,10 @@ env_variables: Dict[str, Callable[[], Any]] = {
     lambda: int(os.getenv("VLLM_ASCEND_ENABLE_NZ", 1)),
     # Decide whether we should enable CP parallelism.
     "VLLM_ASCEND_ENABLE_CONTEXT_PARALLEL":
-    lambda: bool(int(os.getenv("VLLM_ASCEND_ENABLE_CONTEXT_PARALLEL", '0')))
+    lambda: bool(int(os.getenv("VLLM_ASCEND_ENABLE_CONTEXT_PARALLEL", '0'))),
+    # Whether to anbale dynamic EPLB
+    "DYNAMIC_EPLB":
+    lambda: os.getenv("DYNAMIC_EPLB", "false").lower(),
 }
 
 # end-env-vars-definition