ci: fix print-bosh-job-spec-diff exec path
ci: fix print-bosh-job-spec-diff exec path

fixes:
```
Backend error: Exit status: 500, message: {"Type":"ExecutableNotFoundError","Message":"exec failed: container_linux.go:346: starting container process caused \"exec: \\\"routing-release-ci/ci/tasks/print-bosh-job-diff/task\\\": stat routing-release-ci/ci/tasks/print-bosh-job-diff/task: no such file or directory\"","Handle":"","ProcessID":"","Binary":""}
```

cloudfoundry-incubator/cf-routing-release,cloudfoundry-incubator/cf-routing-release,cloudfoundry-incubator/cf-routing-release