ci: only run bulk units for material changes #17106

Job	Run time
generate-readme	1m 6s
gapic-bom	1m 45s
owlbot-yaml	1m 1s
package_name_check	1m 1s
group_id_check_for_maps_libraries	7m 34s
owlbot-py	1m 16s
gitignore_correctness	1m 7s
root-pom	1m 45s
	16m 35s

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