vagrant: fix gem extension build warnings

[ynishinaka]

Same as #431923.

The problem is that every invocation of vagrant emits following warnings:

Ignoring debug-1.9.2 because its extensions are not built. Try: gem pristine debug --version 1.9.2
Ignoring racc-1.7.3 because its extensions are not built. Try: gem pristine racc --version 1.7.3
Ignoring rbs-3.4.0 because its extensions are not built. Try: gem pristine rbs --version 3.4.0
Ignoring debug-1.9.2 because its extensions are not built. Try: gem pristine debug --version 1.9.2
Ignoring racc-1.7.3 because its extensions are not built. Try: gem pristine racc --version 1.7.3
Ignoring rbs-3.4.0 because its extensions are not built. Try: gem pristine rbs --version 3.4.0
Ignoring debug-1.9.2 because its extensions are not built. Try: gem pristine debug --version 1.9.2
Ignoring racc-1.7.3 because its extensions are not built. Try: gem pristine racc --version 1.7.3
Ignoring rbs-3.4.0 because its extensions are not built. Try: gem pristine rbs --version 3.4.0

This problem is also mentioned at #434750 (comment).

By using ruby 3.4, the warnings dissappear.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
    • on top of vagrant: fix build failure on darwin #437371
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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