Add support to specify build architecture for deb release

[animator]

Fix for Linux deb packager highlighted in #116

Users can now specify build architecture - arm64, amd64 (default if not specified)
in the make_config.yaml file via build_arch option (same option name as used by rpm maker)

Tested it by building foss42/api-dash linux builds.

[animator]

No idea why lint / format (pull_request) failed 🤷🏻‍♀️

[lijy91]

I think the arch should not be specified by the user, but should be obtained from the current system, just like this.

flutter_distributor/packages/flutter_app_builder/lib/src/builders/linux/build_linux_result.dart

Lines 18 to 28 in 6daea51

	String get arch {
	if (_arch == null) {
	ProcessResult r = Process.runSync('uname', ['-m']);
	if ('${r.stdout}'.trim() == 'aarch64') {
	_arch = 'arm64';
	} else {
	_arch = 'x64';
	}
	}
	return _arch!;
	}

[animator]

The reason I took this route is because it requires minimum code changes and gives more control to the user as arm64 and amd64 are just two architectures, there are plenty more which the current getter in builders/linux/build_linux_result.dart does not handle. Let me know your thoughts.