restrict backend ci checks to ci changes #12640

Job	Run time
build_ee_312	0s
build	7m 38s
build_ee	7m 18s
build_ee_nsjail	1m 58s
build_ee_full	0s
build_ee_cuda	0s
build_ee_slim	0s
run_integration_test	3m 12s
attach_amd64_binary_to_release	0s
build_full	0s
build_slim	0s
build_ee_reports_privately	1m 21s
publish_ecr_s3	1m 46s
tag_latest	20s
tag_latest_ee	21s
verify_ee_image_vulnerabilities	0s
	23m 54s

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