Zero $flag on call

[Dentosal]

Spec PR: https://github.com/FuelLabs/fuel-specs/pull/585

Called contracts shouldn't inherit the $flag register.

Checklist

• [x] Breaking changes are clearly marked as such in the PR description and changelog
• [x] New behavior is reflected in tests
• [x] If performance characteristic of an instruction change, update gas costs as well or make a follow-up PR for that
• [x] The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• [x] I have reviewed the code myself
• [x] I have created follow-up issues caused by this PR and linked them here

After merging, notify other teams

• [ ] Sway compiler