Update CMake install test to use shared libraries

This should help us catch problems that come up when libprotobuf and libprotoc
are built as shared libraries. The motivating example was that we recently had
build failures due to `PROTOBUF_EXPORT` being missing from a symbol that needed
it, but none of our existing tests caught this. (Technically this test wouldn't
catch it either since that particular issue affected C++17 only, but at least
this should help with similar problems.)

PiperOrigin-RevId: 575218922

.github/workflows/test_cpp.yml

@@ -172,7 +172,7 @@ jobs:
           image: us-docker.pkg.dev/protobuf-build/containers/test/linux/cmake:3.13.3-63dd26c0c7a808d92673a3e52e848189d4ab0f17
           credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
           command: >-
-            /install.sh -DCMAKE_CXX_STANDARD=14 ${{ env.SCCACHE_CMAKE_FLAGS }} -Dprotobuf_USE_EXTERNAL_GTEST=ON -Dprotobuf_ABSL_PROVIDER=package \&\&
+            /install.sh -DCMAKE_CXX_STANDARD=14 ${{ env.SCCACHE_CMAKE_FLAGS }} -Dprotobuf_USE_EXTERNAL_GTEST=ON -Dprotobuf_ABSL_PROVIDER=package -Dprotobuf_BUILD_SHARED_LIBS=ON \&\&
             /test.sh
             ${{ env.SCCACHE_CMAKE_FLAGS }}
             -Dprotobuf_REMOVE_INSTALLED_HEADERS=ON