add keep-build-dirs flag #744

Job	Run time
Flatpak Builder Stop At	1m 28s
Flatpak Builder with Dedicated Path Params	1m 35s
Lint	25s
Flatpak Builder (aarch64, use-cache, cache-restored)	3m 6s
Flatpak Builder (x86_64, no-cache, no-cache-restored)	1m 35s
Flatpak Builder (x86_64, use-cache, no-cache-restored)	1m 38s
Flatpak Builder (x86_64, use-cache, cache-restored)	1m 30s
Tests (18.x)	21s
Tests (20.x)	20s
Flatpak Builder Cache Hit	1m 28s
	13m 26s

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