CMake GitHub workflow: enable shelf3, so we run their tests.

* .github/workflows/cmake.yml
  (env): SVN_EXPERIMENTAL_COMMANDS=shelf3


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1922646 13f79535-47bb-0310-9956-ffa450edef68

Author: rinrab

.github/workflows/cmake.yml

@@ -88,6 +88,7 @@ jobs:
     env:
       VCPKG_BINARY_SOURCES: "clear;x-gha,readwrite"
       CMAKE_GENERATOR: ${{ matrix.cmake_generator }}
+      SVN_EXPERIMENTAL_COMMANDS: shelf3
 
     steps:
       - name: Prepare Environment (Windows)