Check if BUILD_CONFIG_DATA_DIR exists before mounting #1211

Job	Run time
clickable (Debug, amd64, glew)	11m 14s
clickable (Debug, amd64, sdl)	10m 32s
clickable (Debug, armhf, glew)	10m 17s
clickable (Debug, armhf, sdl)	9m 14s
clickable (Release, amd64, glew)	10m 33s
clickable (Release, amd64, sdl)	9m 56s
clickable (Release, arm64, glew)	10m 35s
clickable (Release, arm64, sdl)	10m 38s
clickable (Release, armhf, glew)	9m 52s
clickable (Release, armhf, sdl)	9m 0s
	1h 41m 51s

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