Fix when writing to directory that contains "tsv" in its name

[hoechenberger]

This is the "quick & dirty" fix described in #832.

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"

[codecov-commenter]

Codecov Report

Merging #833 (30bafee) into main (11e3efc) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #833   +/-   ##
=======================================
  Coverage   94.05%   94.05%           
=======================================
  Files          23       23           
  Lines        3046     3046           
=======================================
  Hits         2865     2865           
  Misses        181      181           

Impacted Files	Coverage Δ
mne_bids/write.py	96.20% <100.00%> (ø)

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