Fix fork_join exception fallback storage

[fallintoplace]

This fixes the exception fallback in exec::fork_join.

When caching a potentially throwing completion argument, fork_join currently tries to emplace the exception result through _cache_._results_. The cache is already a __results_storage, so that member does not exist and the template branch fails to compile when instantiated.

The fix emplaces the set_error(std::exception_ptr) tuple directly into _cache_.

The regression test uses a value with a potentially throwing copy constructor, returns it by reference from just_from, and verifies that sync_wait rethrows the copy failure.

Tests:

• cmake --build build --target test.exec -j 4
• build/test/exec/test.exec '[adaptors][fork_join]'
• build/test/exec/test.exec

[copy-pr-bot]

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[ericniebler]

/ok to test c4ebe33