build: Fix Systemd unit install location #3070

Job	Run time
macOS 12 native [GOAL install] [GUI] [no depends]	9m 21s
Lint	45s
ARM [GOAL install] [buster]	12m 9s
Win32	2m 50s
Win64	3m 59s
i386 Linux [GOAL install] [focal]	11m 57s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	14m 3s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	13m 26s
x86_64 Linux [ASan] [LSan] [UBSan] [integer] [jammy] [no depends]	4m 47s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	9m 46s
	1h 23m 3s

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