Unfortunately, the packages you install in a Job or Workspaces are not preserved when the run finishes. We are currently planning to support preserving of environment changes in the future, but it can take a while.
Meanwhile, there is a possible workaround for this. You can create a FloydHub dataset which contains the changes to the filesystem from installing dependencies. Then you can mount this dataset inside the Workspace or Job and copy the dependencies in the right path (take a look at the script used by this user for this workaround: create_floyd_base.sh).
Hope that helps.