Improve _get_block_templates_paths performace #723

Job	Run time
Core running from build / Core running from build / Linux	2m 7s
Core running from src / Core running from src / Linux	2m 0s
Core running from build / Core running from build / Windows	3m 36s
Core running from src / Core running from src / Windows	3m 14s
Core running from build / Core running from build / MacOS	4m 8s
Gutenberg running from src / Gutenberg running from src / Linux	7m 20s
Core running from src / Core running from src / MacOS	3m 9s
Gutenberg running from build / Gutenberg running from build / Linux	7m 28s
Gutenberg running from build / Gutenberg running from build / Windows	11m 43s
Gutenberg running from src / Gutenberg running from src / Windows	11m 17s
Gutenberg running from build / Gutenberg running from build / MacOS	11m 57s
Gutenberg running from src / Gutenberg running from src / MacOS	17m 48s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 25m 47s

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