Provide universal binary for macOS

Description

Currently, Gitlab provides two binaries for gitlab-runner for darwin:

This complicates deployment in environments where there is a mix of x86 and arm based Macs.

Create universal binary and distribute it as a single executable on macOS.

This can be achieved by running following command:

lipo -create -output gitlab-runner-darwin-universal gitlab-runner-darwin-amd64 gitlab-runner-darwin-arm64