[Backport 7.63.x] exclude `.git` from copyright linter #69450

Job	Run time
assign-team-label	41s
agenttelemetry-list-change-ack-check	38s
release-note-check	1m 22s
ask-reviews	0s
fetch-labels	2s
skip-qa-check	28s
team-label-check	1s
	3m 12s

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