Problem: fail to build in recent nixpkgs

[yihuang]

Solution:

• fix the expression

👮🏻👮🏻👮🏻 !!!! REFERENCE THE PROBLEM YOUR ARE SOLVING IN THE PR TITLE AND DESCRIBE YOUR SOLUTION HERE !!!! DO NOT FORGET !!!! 👮🏻👮🏻👮🏻

PR Checklist:

• Have you read the CONTRIBUTING.md?
• Does your PR follow the C4 patch requirements?
• Have you rebased your work on top of the latest master?
• Have you checked your code compiles? (make)
• Have you included tests for any non-trivial functionality?
• Have you checked your code passes the unit tests? (make test)
• Have you checked your code formatting is correct? (go fmt)
• Have you checked your basic code style is fine? (golangci-lint run)
• If you added any dependencies, have you checked they do not contain any known vulnerabilities? (go list -json -m all | nancy sleuth)
• If your changes affect the client infrastructure, have you run the integration test?
• If your changes affect public APIs, does your PR follow the C4 evolution of public contracts?
• If your code changes public APIs, have you incremented the crate version numbers and documented your changes in the CHANGELOG.md?
• If you are contributing for the first time, please read the agreement in CONTRIBUTING.md now and add a comment to this pull request stating that your PR is in accordance with the Developer's Certificate of Origin.

Thank you for your code, it's appreciated! :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de1508e) 30.08% compared to head (04a0e88) 30.08%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1031   +/-   ##
=======================================
  Coverage   30.08%   30.08%           
=======================================
  Files         125      125           
  Lines       20039    20039           
=======================================
  Hits         6028     6028           
  Misses      12965    12965           
  Partials     1046     1046           

Flag	Coverage Δ
integration_tests	20.71% <ø> (ø)
integration_tests_byzantine	9.66% <ø> (ø)
integration_tests_gov	10.03% <ø> (ø)
integration_tests_grpc	10.01% <ø> (ø)
integration_tests_ibc	24.81% <ø> (ø)
integration_tests_ledger	9.90% <ø> (ø)
integration_tests_slow	9.90% <ø> (ø)
integration_tests_solomachine	9.69% <ø> (ø)
integration_tests_upgrade	10.01% <ø> (ø)

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