Always ensure that DIST_DIR is set

Set the default value if one isn't provided.

Test: `get_build_var DIST_DIR` after removing the default setting in envsetup.mk
Change-Id: I0cb310fc65f7747c36de14608b61786ef6863fd1

ui/build/config.go

@@ -86,6 +86,9 @@ func NewConfig(ctx Context, args ...string) Config {
 		ret.environ.Set("OUT_DIR", outDir)
 	}
 
+	// Make sure DIST_DIR is set appropriately
+	ret.environ.Set("DIST_DIR", ret.DistDir())
+
 	ret.environ.Unset(
 		// We're already using it
 		"USE_SOONG_UI",