Install `virtual/rust` to avoid version conflicts that happen in case of
rust versions in the SDK being different from those in the new ebuilds.
`/usr/share/catalyst/targets/stage1/stage1-chroot.sh` installs gcc and
its dependencies, including `dev-lang/rust`, while `virtual/rust` does
not get updated. That results in version conflicts between
`virtual/rust` and `dev-lang/rust`. To avoid such an issue, we should
update also `virtual/rust` when building stage1. Since `virtual/rust`
automatically pulls in `dev-lang/rust`, we do not need to explicitly
specify `dev-lang/rust` here.
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