build: explicitly include FindPkgConfig for CMake #3113

Job	Run time
macOS 12 native [GOAL install] [GUI] [no depends]	11m 57s
ARM [GOAL install] [buster]	12m 0s
Lint	1m 6s
Win32	13m 58s
Win64	46m 45s
i386 Linux [GOAL install] [focal]	12m 2s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	16m 37s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	15m 36s
x86_64 Linux [ASan] [LSan] [UBSan] [integer] [jammy] [no depends]	19m 32s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	29m 5s
	2h 58m 38s

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