chore: Pin flutter in the Netlify build

[robinlinden]

Before this, we were using the master branch of flutter here, leading to the preview not matching what you'd see when building bTox locally.

This was pretty obvious in the last PR where the master branch of flutter has fully migrated to the Material 3 theme whereas our version still has it as an opt-in.

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This change is 

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[codecov[bot]]

Codecov Report

Merging #35 (77a2b7e) into master (603d84e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   41.17%   41.17%           
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  Files           7        7           
  Lines         221      221           
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  Hits           91       91           
  Misses        130      130