[libc++] Disable modules with the bootstrapping build

It turns out that we had never been enabling it anyways, since the
LIBCXX_TEST_PARAMS parameter was not being passed from the bootstrapping
build to the libc++ and libc++abi builds. Furthermore, it looks like the
per-target include directories used by the bootstrapping build by default
are incompatible with our current modulemap, since __config_site doesn't
live in the directory that our modulemap claims.

This disables modules in our bootstrapping CI job to unblock D121822,
but we should work on fixing the underlying issue once we're able to
pass those configuration options to our bootstrapping build.