…2110)

PR pypa#2073 (workspace support) introduced a separate `additional_dependencies`
list on `EnvironmentInterface` and changed `prepare_build_environment` to
extend that list with the dynamic build-hook dependencies discovered by
`get_required_build_deps` (e.g. project runtime deps when a build hook sets
`require-runtime-dependencies = true`).

Bug: by the time `prepare_build_environment` extends
`additional_dependencies`, the cached property chain that consumes it:
`dependencies_complex` -> `all_dependencies_complex` -> `missing_dependencies`
has already been populated by the preceding `prepare_environment` call (via
`dependency_hash()` and `dependencies_in_sync()`) with the *empty*
`additional_dependencies` snapshot.

The second `sync_dependencies()` call then early-returns because the
cached `missing_dependencies` reflects the pre-mutation deps which have
just been installed — so the runtime deps never get installed and the
build hook fails with `ModuleNotFoundError`.

Before PR pypa#2073, this worked because the old code mutated the cached
`dependencies` list in place and the old `sync_dependencies` reinstalled
the entire list via pip — there was no cached `missing_dependencies`
short-circuit.

Fix: add `EnvironmentInterface.add_additional_dependencies()` which extends
`additional_dependencies` and invalidates the dependent cached properties
(`dependencies_complex`, `dependencies`, `all_dependencies_complex`,
`all_dependencies`, `missing_dependencies`). Update `prepare_build_environment`
to use it.

Add a regression test that primes the cache before extending and asserts
the new dep is visible.