[CMake] Always use builder's runtime for building Swift code in Linux
Some non-stdlib thing e.g. swift-backtrace still might be built before
the runtime is built. For building Swift code in Linux "hosttools",
always set 'LD_LIBRARY_PATH' to the runtime in the builder.
(cherry picked from commit 0aa0aac72e923b53801b32bb09fc1149185a256f)
(cherry picked from commit d8a12651e6955a6e46d59106b39ceabde02bb8b5)