Scripts: Fix Octez source by adding [rust-toolchain] to [octez-source-content]

What

Add rust-toolchain to Octez source (script-inputs/octez-source-content)

Why

make octez is broken for Octez tarballs, generated from octez-source-content.

See https://gitlab.com/tezos/tezos/-/jobs/6875340921

How

Just added rust-toolchain to script-inputs/octez-source-content

Manually testing the MR

See https://gitlab.com/nomadic-labs/tezos/-/pipelines/1295218250

• ./scripts/ci/restrict_export_to_octez_source.sh
• ./scripts/ci/create_octez_tarball.sh octez
• mv octez.tar.bz2 ../
• cd ../
• tar xf octez.tar.bz2
• cd octez/
• eval $(opam env)
• make octez

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR