ci: cache correct static folder #3398

Job	Run time
File changes / List changed files	8s
Linting / CSS	40s
Build app (or restore from cache)	2m 59s
Linting / TypeScript	53s
Linting / JavaScript	23s
Linting / Elixir	1m 46s
Unit tests / JavaScript / Mocha	55s
Formatting / JavaScript & TypeScript	31s
Unit tests / JavaScript & TypeScript / Jest	2m 32s
Type checks / Elixir	4m 30s
Type checks / TypeScript	50s
Formatting / Elixir	25s
Integration tests / Elixir	1m 17s
Unit tests / Elixir / --exclude wallaby --cover	4m 3s
	21m 52s

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