vcpkg detects the compiler hash before the CMake toolchain file loads,
so -DCMAKE_C_COMPILER=gcc-14 on the cmake line was too late -- vcpkg
picked up /usr/bin/c++ (gcc-13 on Ubuntu 24.04) and computed an ABI
hash that did not match the gcc-14 entries baked into the image's
/opt/vcpkg-cache-gcc, restoring 0 packages and rebuilding all 102
ports from source. Export CC/CXX in the step env to match the
Dockerfile's vcpkg-builder-gcc stage.