Use `defaultdict` to prevent KeyError (#4319) #3154

Job	Run time
Check backend needs building	17s
Check frontend needs building	13s
Check pretix needs building	7s
Build pretix	0s
Build backend / Build	2m 2s
Deploy backend	42s
Wait backend deployment	1m 23s
Build frontend	0s
Deploy frontend	32s
Wait frontend deployment	9s
	5m 25s

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