Ports: Export CMAKE_BUILD_PARALLEL_LEVEL for ports scripts

When using cmake --build, CMake will look for this environment variable
to enable parallelism. The Zig port, for example, uses cmake --build,
and will otherwise use a single core if cmake selects Make as the build
system. This should help with all ports which use cmake --build.

Ports/.port_include.sh

@@ -9,6 +9,7 @@ fi
 unset SERENITY_STRIPPED_ENV
 
 export MAKEJOBS="${MAKEJOBS:-$(nproc)}"
+export CMAKE_BUILD_PARALLEL_LEVEL="$MAKEJOBS"
 
 buildstep() {
     local buildstep_name=$1