[#26] Fix `validate cabal files` CI step

[dcastro]

Description

Problem: The validate cabal files build step hasn't been working ever since we added it. We can see this message in the logs:

./scripts/validate-cabal-files.sh: line 39: diff: command not found

The reason why the build step did not actually ever fail (thus, it was a false negative) is because we temporarily use set +e to mask all non-zero exit codes.

Solution: add the missing diffTools to the nix shell.

Related issue(s)

Fixed part of #26

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