ci: don't run more than once per PR branch (#1104) #2258

Job	Run time
integration (kong:1.4.3)	1m 16s
integration (kong:1.5.1)	1m 50s
integration (kong:2.0.5)	1m 57s
integration (kong:2.1.4)	1m 23s
integration (kong:2.2.2)	1m 21s
integration (kong:2.3.3)	1m 38s
integration (kong:2.4.1)	1m 28s
integration (kong:2.5.1)	1m 24s
integration (kong:2.6.0)	1m 23s
integration (kong:2.7)	1m 53s
integration (kong:2.8)	1m 19s
integration (kong:3.0)	1m 32s
integration (kong:3.1)	2m 3s
integration (kong:3.2)	1m 27s
integration (kong:3.3)	2m 16s
integration (kong/kong:master-alpine)	1m 54s
	26m 4s

Provide feedback