refactor: Do not assume there is only one workspace target in app.py

While working with workspaces in _frontend/app.py, we currently assume
that there is only workspace in the pippeline. This limits our ability
to support multiple workspaces in a given command.

Remove this assumption from _frontend/app.py which will allow for the
possibility of supporting multiple workspaces from _frontend/cli.py in
future. This commit does not change the behavior of these commands
though as the target is instead passed as an argument.