build: use gcc-12 for flatc build

[mhilton]

After the letest debian release the default gcc doesn't compiel flatc. Use the older gcc-12 version instead.

Checklist

Dear Author 👋, the following checks should be completed (or explicitly dismissed) before merging.

• ✏️ Write a PR description, regardless of triviality, to include the value of this PR
• 🔗 Reference related issues
• 🏃 Test cases are included to exercise the new code
• 🧪 If new packages are being introduced to stdlib, link to Working Group discussion notes and ensure it lands under experimental/
• 📖 If language features are changing, ensure docs/Spec.md has been updated

Dear Reviewer(s) 👋, you are responsible (among others) for ensuring the completeness and quality of the above before approval.