GHA/checkurls: add dry run on push

To verify if the basics work.

Downside is that the scheduled (live) runs are intermixed with the dry
runs and less obvious to find in the default list:
https://github.com/curl/curl/actions/workflows/checkurls.yml

This URL filters for scheduled runs only:
https://github.com/curl/curl/actions/workflows/checkurls.yml?query=event%3Aschedule

Seems fine, because we're only interested in red runs.

Closes #19917

.github/workflows/checkurls.yml

@@ -5,6 +5,13 @@
 name: 'URLs'
 
 'on':
+  push:
+    branches:
+      - master
+      - '*/ci'
+  pull_request:
+    branches:
+      - master
   schedule:
     - cron: '10 5 * * *'
 
@@ -16,7 +23,7 @@ permissions: {}
 
 jobs:
   linkcheck:
-    if: ${{ github.repository_owner == 'curl' }}
+    if: ${{ github.repository_owner == 'curl' || github.event_name != 'schedule' }}
     name: 'linkcheck'
     runs-on: ubuntu-latest
     steps:
@@ -24,5 +31,10 @@ jobs:
         with:
           persist-credentials: false
 
+      - name: 'mdlinkcheck (dry run)'
+        if: ${{ github.event_name != 'schedule' }}
+        run: ./scripts/mdlinkcheck --dry-run
+
       - name: 'mdlinkcheck'
+        if: ${{ github.event_name == 'schedule' }}
         run: ./scripts/mdlinkcheck