[CI] Add workfolw_dispatch for nightly image build (#6269)

### What this PR does / why we need it?
Currently, the nightly image is built at 20 PM and 23 PM UTC+8. Due to
some timeliness requirements, we need to add a new trigger method for
nightly image builds.
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.14.1
- vLLM main:
d68209402d

Signed-off-by: wangli <wangli858794774@gmail.com>

.github/workflows/schedule_nightly_image_build.yaml

@@ -4,6 +4,7 @@ on:
   schedule:
     # UTC+8: 20pm, 23pm
     - cron: '0 12,15 * * *'
+  workflow_dispatch: 
 
 # This workflow builds and pushes Docker images for nightly-ci
 # It will be built base on the quay.io/ascend/vllm-ascend:main