Don't fail on non-existent plugin directory. #236

Job	Run time
aarch64 appimage	7m 56s
i386 appimage	7m 13s
x86_64 coverage	6m 14s
armhf appimage	8m 16s
x86_64 appimage	6m 41s
Create release and upload artifacts	11s
	36m 31s

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