Subversion isn't installed by default on GHA runners.

* .github/workflows/detect-backport-conflicts.yml
  (steps): Add a step to install Subversion


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1924123 13f79535-47bb-0310-9956-ffa450edef68

Author: dsahlberg-apache-org

.github/workflows/detect-backport-conflicts.yml

@@ -37,6 +37,12 @@ jobs:
     name: Detect conflicts
 
     steps:
+      - name: Install dependencies (Linux, apt-get)
+        run: >
+          sudo apt-get update &&
+          sudo apt-get install
+          subversion
+          
       - name: Use LF for Git checkout
         run: |
           git config --global core.autocrlf false