Update nix flake inputs structure and add dependency update workflow for rv-nix-tools

[juliankuners]

This pull request adapts the nix flake inputs structure regarding nixpkgs and rv-nix-tools. In particular, nixpkgs is now specified for inputs and transitive inputs by following an explicitly specified revision of rv-nix-tools. More thorough reasoning for this change is outlined in documentation added by this PR runtimeverification/rv-nix-tools#14. This change is done to make nixpkgs properly updatable by CI.

In addition, this pull requests adapts the update-deps.yml workflow to also update rv-nix-tools that is used to implicitly specify the revision of nixpkgs.

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Review checklist

The author performs the actions on the checklist. The reviewer evaluates the work and checks the boxes as they are completed.

• Summary. Write a summary of the changes. Explain what you did to fix the issue, and why you did it. Present the changes in a logical order. Instead of writing a summary in the pull request, you may push a clean Git history.
• Documentation. Write documentation for new functions. Update documentation for functions that changed, or complete documentation where it is missing.
• Tests. Write unit tests for every change. Write the unit tests that were missing before the changes. Include any examples from the reported issue as integration tests.
• Clean up. The changes are already clean. Clean up anything near the changes that you noticed while working. This does not mean only spatially near the changes, but logically near: any code that interacts with the changes!