Use different icon color for nightlies on macOS

Currently, the stable release and nightly KiCad apps on macOS appear the same in the dock:

image

It would be nice if these had different colors to differentiate them. As an example, here's how VS code distinguishes the "insiders" version from the stable release on macOS:

image

KiCad Version

Application: KiCad

Version: (6.99.0-4200-gde21eb5268), release build

Libraries:
	wxWidgets 3.1.5
	FreeType 2.12.1
	HarfBuzz 5.3.1
	FontConfig 2.14.0
	libcurl/7.84.0 (SecureTransport) LibreSSL/3.3.6 zlib/1.2.11 nghttp2/1.47.0

Platform: macOS Version 13.0 (Build 22A380), 64 bit, Little endian, wxMac

Build Info:
	Date: Nov  4 2022 22:12:21
	wxWidgets: 3.1.5 (wchar_t,wx containers)
	Boost: 1.80.0
	OCC: 7.6.3
	Curl: 7.64.1
	ngspice: 37
	Compiler: Clang 12.0.0 with C++ ABI 1002

Build settings:
	KICAD_SPICE=ON