ci: fix build on runners with CMake 4.0

Lately, CMake 4.0 was released, and some GitHub Actions runners started
to use it. CMake 4.0 does not support versions older than 3.5 anymore,
and we fetch MsgPuck library for the tests that requires 2.8 minimum
version.

Let's override minimum required version with CMake option in order to
generate build files of MsgPuck successfully.

Author: drewdzzz

.github/actions/build-tntcxx/action.yml

@@ -29,6 +29,7 @@ runs:
         -DTNTCXX_ENABLE_SANITIZERS=${{ inputs.enable-sanitizers }} \
         -DCMAKE_CXX_STANDARD=${{ inputs.cxx-standard }} \
         -DCMAKE_C_COMPILER=${{ inputs.c-compiler }} \
-        -DCMAKE_CXX_COMPILER=${{ inputs.cxx-compiler }}
+        -DCMAKE_CXX_COMPILER=${{ inputs.cxx-compiler }} \
+        -DCMAKE_POLICY_VERSION_MINIMUM=3.5
         make -j
       shell: bash