main: improve XDG_RUNTIME_DIR handling

1. Variable xdgRuntimeDir is only checked to be non-empty. Change it to
   a boolean.

2. Refactor so that os.Getenv is not called twice.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

rootless_linux.go

@@ -52,9 +52,6 @@ func shouldUseRootlessCgroupManager(context *cli.Context) (bool, error) {
 }
 
 func shouldHonorXDGRuntimeDir() bool {
-	if os.Getenv("XDG_RUNTIME_DIR") == "" {
-		return false
-	}
 	if os.Geteuid() != 0 {
 		return true
 	}