Copy soong variables to dist

Per aosp/2183398, out/soong/soong.variables provide some useful information for debugging purpose.
This change enables copying out/soong/soong.variables to out/dist/soong_ui/soong/soong.variables in dist mode, similar to build.ninja.gz in the same directory.
Test confirmed that the content of the out/soong/soong.variables is
equal to that of out/dist/soong_ui/soong/soong.variables.

Test: m dist $target &&
	diff out/soong/soong.variables out/dist/soong_ui/soong/soong.variables
Change-Id: I1bfa53a8e116c49fda2cc9a7252f4da0c59ce5b5