ci: Update to `actions/checkout@v4` from v3. #174

Job	Run time
stable, Linux, static linking, no pkg-config	1m 25s
stable, Linux, shared library	39s
stable, iOS, static library	1m 16s
stable, macOS, shared library	38s
stable, macOS, static library	1m 34s
stable, Emscripten, static library	1m 25s
stable, Windows, static library	1m 58s
Result	1s
	8m 56s

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