dependabot: explicitly exclude rust-vmm deps from firecracker group

[roypat]

Apparently, if a rust-vmm dep (currently vmm-sys-util) is included in a
PR for the firecracker group once, it will indefinitely be stuck in
there. So explicitly exclude these deps from that group, in hopes of
getting dependabot to understand that it should split the current PR
into two.

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PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

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• This functionality cannot be added in rust-vmm.