ci: fix flutter version on post-merge workflow

[Kirpal]

Description

Type of Change

• [ ] ✨ New feature (non-breaking change which adds functionality)
• [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [x] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🗑️ Chore

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (a8d1f0e) compared to base (bc269cc). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #89 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 13 13 Lines 556 558 +2 ========================================= + Hits 556 558 +2 ``` | [Impacted Files](https://codecov.io/gh/Betterment/alchemist/pull/89?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Betterment) | Coverage Δ | | |---|---|---| | [lib/src/blocked\_text\_image.dart](https://codecov.io/gh/Betterment/alchemist/pull/89?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Betterment#diff-bGliL3NyYy9ibG9ja2VkX3RleHRfaW1hZ2UuZGFydA==) | `100.00% <100.00%> (ø)` | | | [lib/src/utilities.dart](https://codecov.io/gh/Betterment/alchemist/pull/89?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Betterment#diff-bGliL3NyYy91dGlsaXRpZXMuZGFydA==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Betterment). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Betterment)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.