chore: fix OS detection with uname -s

[mdqst]

Description

I noticed that the uname command in the script could return different values on different systems, leading to potential issues when detecting macOS. To ensure more reliable OS detection, I've updated the check to use uname -s, which provides the correct system name and accurately identifies macOS. This change will improve the script's compatibility across various platforms.

How to test

1. Run the script on different operating systems (e.g., macOS, Linux).
2. Ensure that the uname -s check correctly identifies macOS without issues on other systems.
3. Verify that the script runs successfully and builds/upload assets as expected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Create pull request as draft initially, unless it's complete.
• Add tests to cover changes as needed.
• Update documentation as needed.
• If there is a related Issue, mention it in the description.
• I have read the [guidelines](https://developer.trustwallet.com/wallet-core/newblockchain#integration-criteria) for adding a new blockchain.

[satoshiotomakan]

Hi @mdqst, could you please replace all uname usages within tools directory?

[mdqst]

satoshiotomakan

satoshiotomakan, sure.