remove default value for inputs.arch - breaks auto-detection

[fclante]

remove default value for inputs.arch because it breaks the auto-detection fallback to getArch()

Before:

If no input value for arch is is supplied it auto-fallback to 'x64' which shunts getArch() method entirely.

   const arch = core.getInput('arch', { required: false }) || getArch();

After:

If no value is supplied for arch it falls back to getArch()

const getArch = () => {
  switch(process.arch) {
    case 'x64':
      return 'x64'
    case 'arm64':
      return 'aarch64'
    default:
      throw new Error(`Unsupported architecture: ${process.arch}`);
  }
}

[fclante]

@Warchant Hi - Did you have a chance to look at this?