Map all generated input roots into legacy doctest runfiles (#4271)

Legacy doctests run from a runfiles directory, but the launcher only
strips output roots found through crate and C++ library providers. A
generated rustc environment file built in an execution configuration
retains a `bazel-out/...` path and fails before rustdoc starts.

Collect the nonempty roots of every declared action input instead. This
also covers generated compiler/linker tools and keeps source paths
intact. The new regression executes a doctest using an environment file
forwarded through an execution transition; it fails with a missing-file
error on main and passes with this change.

Validation on macOS with Bazel 9.2.0: 17 owning tests pass on this
standalone branch. This extends the C++ runtime-root handling from
#4220. Windows execution was not tested locally.

Closes #4288.