Fix prebuild script for windows

[takameyer]

What, How & Why?

The publish action was outputting windows prebuilds into prebuilds\realm-v12.0.0-rc.2-napi-v6-win32-%PREBUILD_ARCH%.tar.gz, which shows a mismatch between unix based envs and windows env. The tool cross-env was being used to ensure this method worked, but the cross-env-shell variant was needed so that the env was made available in the shell executing the build script and not just the script itself.

This will fix any issues Windows users have had installing v12 in node environments.

This closes #6011

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 📝 Update COMPATIBILITY.md
• 🚦 Tests
• 🔀 Executed flexible sync tests locally if modifying flexible sync
• 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• 📱 Check the React Native/other sample apps work if necessary
• 📝 Public documentation PR created or is not necessary
• 💥 Breaking label has been applied or is not necessary

If this PR adds or changes public API's:

• typescript definitions file is updated
• jsdoc files updated