[github-actions] add write permissions for geekyeggo/delete-artifact … #1540

Job	Run time
backbone-router	13m 51s
BR (mDNSResponder, TREL=0)	31s
BR (mDNSResponder, TREL=1)	1s
BR MATN (mDNSResponder, TREL=0)	11m 57s
BR LowPower (mDNSResponder, TREL=0)	1s
BR nat64 (mDNSResponder, TREL=0)	1s
BR (avahi, TREL=0)	10m 53s
BR (avahi, TREL=1)	12m 4s
BR core-firewall (avahi, TREL=0)	13m 7s
upload-coverage	1s
delete-coverage-artifacts	1s
	1h 2m 28s

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