FIX: manually marked bad channels must be excluded in find bads

[sappelhoff]

PR Description

Sometimes we know in advance that a channel is bad and we want to explicitly exclude it. This is possible by marking it as bad before finding additional bad channels.

Merge Checklist

• the PR has been reviewed and all comments are resolved
• all CI checks pass
• (if applicable): the PR description includes the phrase closes #<issue-number> to automatically close an issue
• (if applicable): the changes are documented in the changelog changelog.rst
• (if applicable): new contributors have added themselves to the authors list in the CITATION.cff file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (65976a8) to head (00692e1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #168   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files           7        7           
  Lines         721      723    +2     
=======================================
+ Hits          711      713    +2     
  Misses         10       10           

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