ci: bump RIOT_BRANCH to latest release 2025.04

Author: mguetschow

.github/workflows/build.yml

@@ -36,7 +36,7 @@ jobs:
     name: Build and Test
     runs-on: ubuntu-latest
     env:
-      RIOT_BRANCH: '2025.01-branch'
+      RIOT_BRANCH: '2025.04-branch'
       VERSION_TAG: '2025.04'
       DOCKER_REGISTRY: "${{ secrets.DOCKER_REGISTRY || 'local' }}"