chore: add permanent and conditional file exclusions #18675

Job	Run time
owlbot-yaml	1m 1s
gapic-bom	1m 40s
group_id_check_for_maps_libraries	2m 59s
package_name_check	1m 0s
generate-readme	1m 7s
owlbot-py	1m 10s
gitignore_correctness	56s
root-pom	1m 45s
	11m 38s

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