github: added explicit do not merge label to label check (matplotlib#29494)

* added explicit do not merge label to block merge check

* changed label name

Author: story645

.github/workflows/do_not_merge.yml

@@ -15,14 +15,16 @@ jobs:
     env:
       has_tag: >-
          ${{contains(github.event.pull_request.labels.*.name, 'status: needs comment/discussion') ||
-            contains(github.event.pull_request.labels.*.name, 'status: waiting for other PR')}}
+            contains(github.event.pull_request.labels.*.name, 'status: waiting for other PR')     ||
+            contains(github.event.pull_request.labels.*.name, 'DO NOT MERGE') }}
     steps:
       - name: Check for label
         if: ${{'true' == env.has_tag}}
         run: |
           echo "This PR cannot be merged because it has one of the following labels: "
           echo "* status: needs comment/discussion"
           echo "* status: waiting for other PR"
+          echo "* DO NOT MERGE"
           exit 1
       - name: Allow merging
         if: ${{'false' == env.has_tag}}