Remove unnecessary permissions from shipwright-build-webhook #2960

Job	Run time
unit	2m 43s
integration (v1.25.11)	25m 8s
e2e (v1.25.11)	27m 22s
test-docker-build (base)	31s
integration (v1.28.0)	25m 28s
e2e (v1.28.0)	27m 57s
test-docker-build (git)	1m 19s
test-docker-build (image-processing)	1m 0s
test-docker-build (waiter)	48s
	1h 52m 16s

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