ci: install pnpm first #19

Job	Run time
lint	1m 7s
test-browser-windows (chrome, chromium)	6m 31s
test-browser (chrome, chromium)	3m 36s
test-browser-windows (edge, webkit)	6m 35s
test (ubuntu-latest, 18)	11m 2s
test-browser (firefox, firefox)	3m 53s
test (ubuntu-latest, 20)	10m 23s
test-browser (edge, webkit)	3m 25s
test (macos-14, 20)	9m 6s
test-ui	2m 7s
test (windows-latest, 20)	15m 55s
test-ui-e2e (ubuntu-latest)	1m 55s
typecheck	1m 44s
test-ui-e2e (macos-14)	3m 24s
test-ui-e2e (windows-latest)	3m 19s
	1h 24m 2s

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