fix: use the same method to trigger the client

tests/unit/30_client.bats

@@ -23,7 +23,7 @@ teardown() {
 }
 
 @test "(client) no args should print help" {
-  run /bin/bash -c "./contrib/dokku_client.sh | head -1 | grep -E '^Usage: dokku \[.+\] COMMAND <app> \[.+\]'"
+  run bash -c "${BATS_TEST_DIRNAME}/../../contrib/dokku_client.sh | head -1 | grep -E '^Usage: dokku \[.+\] COMMAND <app> \[.+\]'"
   echo "output: "$output
   echo "status: "$status
   assert_success