fix: add missing LABEL_TRIGGER environment variable to prepare step (#209)

The label_trigger input was defined but not passed as an environment variable
to the prepare step, causing it to be undefined in the prepare script.
This adds the missing LABEL_TRIGGER environment variable mapping.

Co-authored-by: taku.tsunose <taku.tsunose@takutsunosenoMacBook-Pro.local>

action.yml

@@ -106,6 +106,7 @@ runs:
       env:
         TRIGGER_PHRASE: ${{ inputs.trigger_phrase }}
         ASSIGNEE_TRIGGER: ${{ inputs.assignee_trigger }}
+        LABEL_TRIGGER: ${{ inputs.label_trigger }}
         BASE_BRANCH: ${{ inputs.base_branch }}
         BRANCH_PREFIX: ${{ inputs.branch_prefix }}
         ALLOWED_TOOLS: ${{ inputs.allowed_tools }}