Themes: Optimize WP_Theme::get_post_templates() for efficiency #28939

Job	Run time
Core running from src / Linux	2m 10s
Core running from build / Linux	2m 22s
Core running from src / MacOS	3m 1s
Core running from build / MacOS	1m 58s
Core running from build / Windows	3m 33s
Core running from src / Windows	3m 25s
Slack Notifications	0s
Failed workflow tasks	0s
	16m 29s

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