fix: Don't empty previous contents of build dir

[mattjohnsonpint]

Description

In the AssemblyScript SDK, the build script wipes the contents of the build directory clean before writing new files. While this may sound good, there are two side effects that appear when using modus dev:

• The .wasm file is removed, then re-added. So the runtime will see it as unregister/register, rather than just re-register.
• Any files the CLI copies to the build folder (namely .env and .env.*) are removed any time a code change is made.

The Go SDK doesn't have this issue, because it doesn't wipe the build directory. It just overwrites as needed.

Removing the file deletion code from the build script will fix the issues and align the two SDKs to have the same behavior.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to this PR