Build libdispatch with its own module cache

Some of the bots in CI run multiple jobs, so we need to be sure
libdispatch compilations do not attempt to share the module cache.

Addresses rdar://68100533

Author: edymtt

utils/build-script-impl

@@ -2319,6 +2319,7 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DCMAKE_CXX_COMPILER:PATH="${CLANG_BIN}/clang++"
                     -DCMAKE_SWIFT_COMPILER:PATH="${SWIFTC_BIN}"
                     -DCMAKE_Swift_COMPILER:PATH="${SWIFTC_BIN}"
+                    -DCMAKE_Swift_FLAGS:PATH="-module-cache-path \"${module_cache}\""
                     -DCMAKE_INSTALL_PREFIX:PATH="$(get_host_install_prefix ${host})"
                     -DCMAKE_INSTALL_LIBDIR:PATH="lib"