Fix: Update Nimble hash for new Nim version

[crStiv]

Changes

• Updated Nimble hash in nix/nimble.nix to match current Nim version
• Added CI workflow to automatically verify hash when nimble.nix is modified

Details

• Fixed incorrect hash in nix/nimble.nix:

  -   hash = "sha256-sa0irAZjQRZLduEMBPf7sHlY1FigBJTR/vIH4ihii/w=";
  +   hash = "sha256-MVHf19UbOWk8Zba2scj06PxdYYOJA6OXrVyDQ9Ku6Us=";

• Added new workflow .github/workflows/check-nim-hash.yml for automatic hash verification on future updates

Related issues

Fixes #6765

Testing

• Verified locally using nix-build nix/nimble.nix
• CI workflow successfully verifies hash correctness

[github-actions]

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Please open your pull request against the unstable branch.
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