fix: make sed work on macOS and Linux

[mdqst]

What ❔

fix sed -i so the script works on both macOS and Linux.

Why ❔

sed -i behaves differently on macOS and Linux.
on macOS it requires a backup suffix, otherwise it breaks.
this change ensures cross-platform compatibility by checking OSTYPE.

Is this a breaking change?

• Yes
• No

Operational changes

none, just updates the script to work correctly on both platforms.

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zkstack dev fmt and zkstack dev lint.