Fix CI by relaxing our MSRV constraint #2689
                
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Recently a transitive dev-dependency published a new version that is incompatible with Nix's MSRV, causing CI to break. That's annoying, because there's really no reason why a crate's dev-dependencies ought to respect the MSRV. Fix Nix's CI by:
What does this PR do
Checklist:
CONTRIBUTING.md