Ensure FileDownload label text updates correctly

[philippjfr]

Previously it would keep appending new labels.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.38%. Comparing base (cd645dd) to head (415f041).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/ui/widgets/test_misc.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7489      +/-   ##
==========================================
- Coverage   82.38%   82.38%   -0.01%     
==========================================
  Files         339      339              
  Lines       50986    50994       +8     
==========================================
+ Hits        42003    42009       +6     
- Misses       8983     8985       +2     

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