fix: 3953 - explicitly using flutter 3.7.12 as 3.10 is available

[monsieurtanuki]

What

• As flutter 3.10 is available, it's being used in github actions.
• But the code is still in 3.7: there are some minor breaking changes.
• Before we agree to upgrade to 3.10, and before we let the early 3.10 versions being fixed, we need to explicitly use 3.7

Part of

• Upgrade to flutter 3.10? #3953