chore(repo): lock the flutter version of pipelines

[esarbanis]

CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

Locks the flutter version in the GH actions

[deven98]

LGTM

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0ddab63) 58.82% compared to head (ea22588) 58.82%.

❗ Current head ea22588 differs from pull request most recent head 7fbafc7. Consider uploading reports for the commit 7fbafc7 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1786   +/-   ##
========================================
  Coverage    58.82%   58.82%           
========================================
  Files          292      292           
  Lines        18009    18009           
========================================
  Hits         10594    10594           
  Misses        7415     7415           

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[deven98]

LGTM