chore(linux): Check existence of `services` directory

When running the integration tests, we provide a test service that
replaces `keyman-system-service`. DBus needs the updated `.service`
files in the `services` directory so that it will use the test
service. This change outputs an error and returns an non-zero
exit code if it can't find the `services` directory, which usually
means that `keyman-system-service` didn't get build.