Add dist_dir to goma trigger

This is done so that prototype tooling can write logs inside the DIST_DIR.

Test: I wrote out all the environment variables being passed to
goma_ctl.py and the output is https://paste.googleplex.com/6663469954760704. It contains both DIST_DIR and other variables.

Change-Id: Ia5c70e58aa1b87db9777c59cf3e5df1c9e20f08c

ui/build/goma.go

@@ -72,6 +72,7 @@ func startGoma(ctx Context, config Config) {
 	}
 
 	cmd := Command(ctx, config, "goma_ctl.py ensure_start", gomaCtl, "ensure_start")
+	cmd.Environment.Set("DIST_DIR", config.DistDir())
 
 	if output, err := cmd.CombinedOutput(); err != nil {
 		ctx.Fatalf("goma_ctl.py ensure_start failed with: %v\n%s\n", err, output)