Make --output-format and --no-user-output mutually exclusive

[Marcono1234]

See #970 (reply in thread)

When used together, execution now fails, as desired:

error: the argument '--output-format <FORMAT>' cannot be used with '--no-user-output'

Usage: cargo msrv verify --output-format <FORMAT> [-- <CUSTOM_CHECK_OPTS>...]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.13%. Comparing base (8c01c59) to head (5c48020).
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #979      +/-   ##
==========================================
+ Coverage   75.09%   75.13%   +0.03%     
==========================================
  Files          80       79       -1     
  Lines        5530     5549      +19     
==========================================
+ Hits         4153     4169      +16     
- Misses       1377     1380       +3     

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