`exec::fork_join` following a `continues_on` does not compile

[romintomasetti]

The following code does not compile as of 624efd6:

  TEST_CASE("fork_join following a continues_on", "[adaptors][fork_join]") {
    std::atomic<int> witness = 0;

    auto make_then = [&witness]() {
      return ::STDEXEC::then([&witness]() noexcept { ++witness; });
    };

    auto sndr = ::STDEXEC::just() | ::STDEXEC::continues_on(::STDEXEC::inline_scheduler{})
              | exec::fork_join(make_then(), make_then());

    ::STDEXEC::sync_wait(std::move(sndr));

    CHECK(witness == 2);
  }

I think the code is theoretically valid. Starting with ::STDEXEC::schedule(::STDEXEC::inline_scheduler{}) is fine though.

Here is the compiler output.