Fix drive cycle duration bug

[MarcBerliner]

Description

Fixes a bug where the final duration of a drive cycle would not be inferred correctly.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.89%. Comparing base (c06726d) to head (3d92bfa).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/pybamm/experiment/step/base_step.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5153      +/-   ##
===========================================
- Coverage    98.90%   98.89%   -0.01%     
===========================================
  Files          316      316              
  Lines        26194    26198       +4     
===========================================
+ Hits         25906    25909       +3     
- Misses         288      289       +1     

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