Stop using DIST_DIR in Soong

We're only using it to distribute files in case of failure, which isn't
well supported currently, but can be handled for now by using the
DIST_DIR environment variable during the command execution.

This was at least one cause that we'd be re-running Soong during every
build server build, as the DIST_DIR values are unique.

Test: m dist
Change-Id: Ibd5e6b6c46695350de80b745bfb6a6aa685033a0

android/variable.go

@@ -228,7 +228,6 @@ type productVariables struct {
 	Product_is_iot *bool `json:",omitempty"`
 
 	DeviceKernelHeaders []string `json:",omitempty"`
-	DistDir             *string  `json:",omitempty"`
 
 	ExtraVndkVersions []string `json:",omitempty"`