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

ui/build/ninja.go

@@ -60,6 +60,8 @@ func runNinja(ctx Context, config Config) {
 		cmd.Environment.AppendFromKati(config.KatiEnvFile())
 	}
 
+	cmd.Environment.Set("DIST_DIR", config.DistDir())
+
 	// Allow both NINJA_ARGS and NINJA_EXTRA_ARGS, since both have been
 	// used in the past to specify extra ninja arguments.
 	if extra, ok := cmd.Environment.Get("NINJA_ARGS"); ok {