serialize: allow variants to be serialized #2885

Job	Run time
macOS 12 native [GOAL install] [GUI] [no depends]	14m 23s
ARM [GOAL install] [buster]	11m 41s
Lint	52s
Win32	13m 40s
Win64	31m 10s
i386 Linux [GOAL install] [focal]	11m 43s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	16m 15s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	14m 9s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	28m 33s
	2h 22m 26s

Provide feedback