Only require `qgis.gui` import when gui is built #32562

Job	Run time
build (22.04, 5, true, OFF, ON, ON, ON, ON, OFF, OFF, OFF, OFF, ON, false)	17m 45s
build (39, 6, true, ON, OFF, ON, ON, OFF, OFF, ON, ON, ON, OFF, false)	28m 43s
clang-tidy (22.04, 5)	6m 17s
Run tests (5, ALL_BUT_PROVIDERS)	32m 15s
Run tests (5, POSTGRES)	6m 16s
Run tests (5, HANA)	13m 25s
Run tests (6, ALL_BUT_PROVIDERS)	33m 36s
Run tests (5, ORACLE, 22.04, binary-for-oracle)	6m 32s
	2h 24m 49s

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