CI: Workaround CMake 4.0.

GitHub appear to have erroneously introduced CMake 4.0 - including
a compatibility break with CMake < 3.5 causing the picotool build
to fail with an error in mbedtls:

    pico-sdk/lib/mbedtls/CMakeLists.txt:23 (cmake_minimum_required):
      Compatibility with CMake < 3.5 has been removed from CMake.

Install a custom CMake version until this is resolved. Or, y'know,
might as well do this forever because we can't trust upstream :/

See: raspberrypi/pico-sdk#2391
And: actions/runner-images#11926

Author: Daft-Freak

.github/workflows/cmake.yml

@@ -55,6 +55,15 @@ jobs:
       with:
         release: '13.3.Rel1'
 
+    # CMake 4.0 - erroneously introduced into GitHub runners? -
+    # removed support for <3.5 and mbedtls hits this
+    # https://github.com/raspberrypi/pico-sdk/issues/2391
+    # https://github.com/actions/runner-images/issues/11926
+    - name: Setup cmake
+      uses: jwlawson/actions-setup-cmake@v2
+      with:
+        cmake-version: '3.31.6'
+
     - name: CMake Version
       run: cmake --version