Improve theme file caching in class-wp-theme-json.php #5927

Job	Run time
Core running from build / Core running from build / Linux	2m 1s
Core running from src / Core running from src / Linux	2m 2s
Core running from build / Core running from build / Windows	3m 9s
Core running from src / Core running from src / Windows	3m 25s
Core running from build / Core running from build / MacOS	2m 10s
Core running from src / Core running from src / MacOS	1m 39s
Gutenberg running from src / Gutenberg running from src / Linux	9m 2s
Gutenberg running from build / Gutenberg running from build / Linux	8m 46s
Gutenberg running from build / Gutenberg running from build / Windows	13m 5s
Gutenberg running from src / Gutenberg running from src / Windows	12m 12s
Gutenberg running from build / Gutenberg running from build / MacOS	7m 6s
Gutenberg running from src / Gutenberg running from src / MacOS	6m 57s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 11m 34s

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